Remove inclusion of Xlib headers in Cogl headers

Xlib headers define many trivially named objects which can later cause
name collision problems when only cogl.h header is included in a program
or library. Xlib headers are now only included through including the
standalone header cogl-xlib.h.

https://bugzilla.gnome.org/show_bug.cgi?id=661174

Reviewed-by: Robert Bragg <robert@linux.intel.com>

examples/cogl-x11-tfp.c

@@ -1,4 +1,5 @@
 #include <cogl/cogl.h>
+#include <cogl/cogl-xlib.h>
 #include <cogl/winsys/cogl-texture-pixmap-x11.h>
 #include <glib.h>
 #include <stdio.h>