pkg/oonf_api: fix conflicting types on msba2

oonf_api fails to compile for the msba2 target 

```
arm-none-eabi-gcc -DRIOT -DENABLE_NAME -DBOARD_MSBA2 -DINIT_ON_START -DENABLE_LEDS -DBOARD_MSBA2 -DCPU_LPC2387 -DMODULE_ARM_COMMON -DMODULE_AUTO_INIT -DMODULE_CC110X_NG -DMODULE_CC110X_SPI -DMODULE_CONFIG -DMODULE_CORE -DMODULE_CPU -DMODULE_DESTINY -DMODULE_GPIOINT -DMODULE_IEEE802154 -DMODULE_LIB -DMODULE_LPC_COMMON -DMODULE_NET_HELP -DMODULE_NET_IF -DMODULE_OLSR2 -DMODULE_OONF_API -DMODULE_OONF_COMMON -DMODULE_OONF_RFC5444 -DMODULE_POSIX -DMODULE_PROTOCOL_MULTIPLEX -DMODULE_PS -DMODULE_RANDOM -DMODULE_RTC -DMODULE_SHELL -DMODULE_SHELL_COMMANDS -DMODULE_SIXLOWPAN -DMODULE_SYS -DMODULE_TIMEX -DMODULE_TRANSCEIVER -DMODULE_UART0 -DMODULE_VTIMER -O2 -Wall -Wstrict-prototypes -mcpu=arm7tdmi-s -gdwarf-2 -std=gnu99 -fno-delete-null-pointer-checks -O2 -Wall -Wstrict-prototypes -mcpu=arm7tdmi-s -gdwarf-2 -DVERSION=\"v0.3.0-3-gf2be\" -I/home/ludwig/RIOT/core/include -I/home/ludwig/RIOT/drivers/include -I/home/ludwig/RIOT/sys/include -I/home/ludwig/RIOT/boards/msba2/include -I/home/ludwig/RIOT/boards/msba2-common/include -I/home/ludwig/RIOT/boards/msba2-common/drivers/include -I/home/ludwig/RIOT/cpu/lpc2387/include -I/home/ludwig/RIOT/cpu/arm_common/include/ -I/home/ludwig/RIOT/cpu/lpc_common/include -I/home/ludwig/RIOT/sys/net/include -I/home/ludwig/RIOT/drivers/cc110x -I/home/ludwig/RIOT/drivers/cc110x_ng/include -I/home/ludwig/RIOT/sys/posix/include -I /home/ludwig/RIOT/pkg/oonf_api/oonf_api/src-api -I/home/ludwig/RIOT/boards/msba2/include -I/home/ludwig/RIOT/boards/msba2-common/include -I/home/ludwig/RIOT/boards/msba2-common/drivers/include -I/home/ludwig/RIOT/sys/include -I/home/ludwig/RIOT/sys/net/include -I/home/ludwig/RIOT/sys/posix/include -I/home/ludwig/RIOT/sys/posix/pnet/include -c rfc5444_reader.c -o /home/ludwig/RIOT/examples/olsr2/bin/msba2/oonf_rfc5444/rfc5444_reader.o
rfc5444_reader.c:129:1: error: conflicting types for 'rfc5444_reader_handle_packet'
 rfc5444_reader_handle_packet(struct rfc5444_reader *parser, uint8_t *buffer, size_t length) {
 ^
In file included from rfc5444_reader.c:48:0:
/home/ludwig/RIOT/pkg/oonf_api/oonf_api/src-api/rfc5444/rfc5444_reader.h:312:12: note: previous declaration of 'rfc5444_reader_handle_packet' was here
 EXPORT int rfc5444_reader_handle_packet(
            ^
```

Always use the `enum rfc5444_result` type to fix that.