Include gl2platform.h in SDL_opengles2.h so we play nice with GLEW.

Based on feedback from Sven Bergstr?m

include/SDL_opengles2.h

@@ -29,6 +29,7 @@
 #include <OpenGLES/ES2/gl.h>
 #include <OpenGLES/ES2/glext.h>
 #else
+#include <GLES2/gl2platform.h>
 #include <GLES2/gl2.h>
 #include <GLES2/gl2ext.h>
 #endif