Include <fcntl.h> for open()

According to POSIX [1] that's where it is defined.

[1] https://pubs.opengroup.org/onlinepubs/009604499/basedefs/fcntl.h.html

helpers/wayland_wrapper/wl-socket.c

@@ -10,6 +10,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>