[gl/cogl.c] #include <gmodule.h> for OS X builds

In unifying the {gl,gles}/cogl.c code recently, moving most of the code into
common/cogl.c the gmodule.h include was also mistakenly moved.

Thanks to Felix Rabe for reporting this issue.

Note: I haven't tested this fix myself, as I'm not set up to be able to
build for OS X

common/cogl.c

@@ -29,7 +29,6 @@
 #include "cogl.h"
 
 #include <string.h>
-#include <gmodule.h>
 #include <math.h>
 #include <stdlib.h>
 

gl/cogl.c

@@ -26,6 +26,7 @@
 #endif
 
 #include <string.h>
+#include <gmodule.h>
 
 #include "cogl.h"