OSMesa: Fix headers included before shared header

The shared header, internal.h, must be the first header included in
every compilation unit of GLFW.

src/osmesa_context.c

@@ -25,13 +25,12 @@
 //
 //========================================================================
 
+#include "internal.h"
+
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
 
-#include "internal.h"
-
-
 static void makeContextCurrentOSMesa(_GLFWwindow* window)
 {
     if (window)