This fixes the following compile-time warning:

cc1: note: someone does not honour COPTS correctly, passed 2 times
	 LTCOMPILE tport_type_connect.lo
cc1: note: someone does not honour COPTS correctly, passed 2 times
	 LTCOMPILE tport_type_ws.lo
cc1: note: someone does not honour COPTS correctly, passed 2 times
	 LTCOMPILE ws.lo
cc1: note: someone does not honour COPTS correctly, passed 2 times
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
Makefile:1465: recipe for target 'ws.lo' failed
make[12]: *** [ws.lo] Error 1

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>