multiudpsink: Add missing socket.h include

Without this include, macro like SO_BINDTODEVICE is not visible and
associated feature gets out-compiled. This also affects the support for
SO_SNDBUF.

gst/udp/gstmultiudpsink.c

@@ -37,6 +37,10 @@
 
 #include <string.h>
 
+#ifdef HAVE_SYS_SOCKET_H
+#include <sys/socket.h>
+#endif
+
 #include "gst/net/net.h"
 #include "gst/glib-compat-private.h"