byteorder: silence warnings in clang

Compare

```
$ git checkout master && CC=clang make -C examples/rpl_udp/ clean all

[…]le_uint32_t byteorder_btoll(be_uint32_t v)
            ^
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:266:13: warning: unused function 'byteorder_btolll'
      [-Wunused-function]
le_uint64_t byteorder_btolll(be_uint64_t v)
            ^
6 warnings generated.
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/routing
In file included from /home/martine/Repositories/RIOT-OS/RIOT/sys/net/routing/etx_beaconing.c:28:
In file included from /home/martine/Repositories/RIOT-OS/RIOT/sys/net/include/sixlowpan/ip.h:31:
In file included from /home/martine/Repositories/RIOT-OS/RIOT/sys/net/include/net_help.h:30:
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:236:13: warning: unused function 'byteorder_ltobs'
      [-Wunused-function]
be_uint16_t byteorder_ltobs(le_uint16_t v)
            ^
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:242:13: warning: unused function 'byteorder_ltobl'
      [-Wunused-function]
be_uint32_t byteorder_ltobl(le_uint32_t v)
            ^
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:248:13: warning: unused function 'byteorder_ltobll'
      [-Wunused-function]
be_uint64_t byteorder_ltobll(le_uint64_t v)
            ^
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:254:13: warning: unused function 'byteorder_btols'
      [-Wunused-function]
le_uint16_t byteorder_btols(be_uint16_t v)
            ^
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:260:13: warning: unused function 'byteorder_btoll'
      [-Wunused-function]
le_uint32_t byteorder_btoll(be_uint32_t v)
            ^
/home/martine/Repositories/RIOT-OS/RIOT/core/include/byteorder.h:266:13: warning: unused function 'byteorder_btolll'
      [-Wunused-function]
le_uint64_t byteorder_btolll(be_uint64_t v)
            ^
6 warnings generated.
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/link_layer/ieee802154
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/hashes
   text	   data	    bss	    dec	    hex	filename
 186873	    644	 291396	 478913	  74ec1	/home/martine/Repositories/RIOT-OS/RIOT/examples/rpl_udp/bin/native/rpl_udp.elf
make: Leaving directory `/home/martine/Repositories/RIOT-OS/RIOT/examples/rpl_udp'
```

to

```
$ git checkout byteorder-silence && CC=clang make -C examples/rpl_udp/ clean all

Previous HEAD position was fd2ec50... Merge pull request #1848 from locicontrols/cc2538-nvic-prio-bits
Switched to branch 'byteorder-silence'
make: Entering directory `/home/martine/Repositories/RIOT-OS/RIOT/examples/rpl_udp'
Building application rpl_udp for native w/ MCU native.
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/cpu/native
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/cpu/native/periph
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/cpu/native/drivers
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/cpu/native/net
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/boards/native
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/boards/native/drivers
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/core
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/drivers
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/auto_init
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/lib
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/ps
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/posix
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/shell
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/shell/commands
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/timex
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/transceiver
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/uart0
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/vtimer
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/link_layer/net_if
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/transport_layer/socket_base
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/transport_layer/udp
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/crosslayer/net_help
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/network_layer/sixlowpan
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/routing/rpl
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/routing
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/net/link_layer/ieee802154
"make" -C /home/martine/Repositories/RIOT-OS/RIOT/sys/hashes
   text	   data	    bss	    dec	    hex	filename
 186889	    644	 291396	 478929	  74ed1	/home/martine/Repositories/RIOT-OS/RIOT/examples/rpl_udp/bin/native/rpl_udp.elf
make: Leaving directory `/home/martine/Repositories/RIOT-OS/RIOT/examples/rpl_udp'
```