Include Xlib.h when using Xlib API

The common EGL testing code uses XOpenDisplay(), which means we need to
explicitly include the Xlib.h header file, instead of relying on
something else stringing it along.

test/egl_common.c

@@ -23,6 +23,7 @@
 
 #include <err.h>
 #include <epoxy/egl.h>
+#include <X11/Xlib.h>
 #include "egl_common.h"
 
 /**