glsl: extension must be declared first

Mesa follows this rule of the standard.

Issue #2037

Shader compile fines now but SSO interface seems to be broken.

bin/shaders/GSdx.fx

@@ -17,7 +17,6 @@
 
 #ifndef SHADER_MODEL
 #define GLSL 1
-#extension GL_ARB_gpu_shader5 : enable
 #else
 #define GLSL 0
 #endif

plugins/GSdx/GSDeviceOGL.cpp

@@ -1528,6 +1528,7 @@ void GSDeviceOGL::DoExternalFX(GSTexture* sTex, GSTexture* dTex)
 		std::string   config_name(theApp.GetConfigS("shaderfx_conf"));
 		std::ifstream fconfig(config_name);
 		std::stringstream config;
+		config << "#extension GL_ARB_gpu_shader5 : require\n";
 		if (fconfig.good())
 			config << fconfig.rdbuf();
 		else