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>

cogl/cogl-xlib.c

@@ -29,7 +29,8 @@
 #include "config.h"
 #endif
 
-#include <cogl.h>
+#include <cogl-xlib.h>
+
 #include <cogl-internal.h>
 #include <cogl-handle.h>
 #include <cogl-context-private.h>