Fix compile of t/btrace2fio on RHEL5/CentOS5

Get these errors:

t/btrace2fio.c:395: undefined reference to `__bswap_32'
t/btrace2fio.c:396: undefined reference to `__bswap_32'
t/btrace2fio.c:397: undefined reference to `__bswap_64'
t/btrace2fio.c:398: undefined reference to `__bswap_64'
t/btrace2fio.c:399: undefined reference to `__bswap_32'
t/btrace2fio.c:400: undefined reference to `__bswap_32'
t/btrace2fio.c:401: undefined reference to `__bswap_32'
t/btrace2fio.c:402: undefined reference to `__bswap_32'
t/btrace2fio.c:403: undefined reference to `__bswap_32'
t/btrace2fio.c:404: undefined reference to `__bswap_16'
t/btrace2fio.c:405: undefined reference to `__bswap_16'

Issue is that os/os-linux.h needs to include byteswap.h.

Signed-off-by: Jens Axboe <axboe@fb.com>