This fixes the following compile-time warning:

making all mod_verto
make[7]: Entering directory '/home/sk/tmp/lede/build_dir/target-mips_24kc_musl-1.1.15/freeswitch-1.8.0/src/mod/endpoints/mod_verto'
  CC       mod_verto_la-mod_verto.lo
  CC       mod_verto_la-ws.lo
ws.c: In function 'hton64':
ws.c:730:14: error: implicit declaration of function '__bswap_64' [-Werror=implicit-function-declaration]
  else return __bswap_64(val);
              ^
cc1: all warnings being treated as errors

Fix by including byteswap.h, which is available on Linux and also
everywhere glibc is used (wpa_supplicant includes this header the same
way).
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>