gsdx:ogl:windows: Fix calling convention mismatch

OpenGL does not use the cdecl calling convention (which is the default
calling convention for GSdx on Windows). Since DebugOutputToFile is used
by OpenGL, it needs to use the same calling convention that OpenGL uses.

This fixes a debug build crash when the OpenGL renderers are used and
debug_opengl is nonzero in the ini.

plugins/GSdx/GSDeviceOGL.h

@@ -472,7 +472,8 @@ class GSDeviceOGL : public GSDevice
 	virtual ~GSDeviceOGL();
 
 	static void CheckDebugLog();
-	static void DebugOutputToFile(GLenum gl_source, GLenum gl_type, GLuint id, GLenum gl_severity, GLsizei gl_length, const GLchar *gl_message, const void* userParam);
+	// Used by OpenGL, so the same calling convention is required.
+	static void APIENTRY DebugOutputToFile(GLenum gl_source, GLenum gl_type, GLuint id, GLenum gl_severity, GLsizei gl_length, const GLchar *gl_message, const void* userParam);
 
 	bool HasStencil() { return true; }
 	bool HasDepth32() { return true; }