[glfw] Include gl.hpp first to avoid redefinition of GLAPIENTRY

Mesa does not check if GLAPIENTRY was defined before before defining
it, what makes the compiler sad. So we make Mesa define it first,
before GLFW.

platform/glfw/glfw_view.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <mbgl/gl/gl.hpp>
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/view.hpp>
 #include <mbgl/map/backend.hpp>