Use proper debug callback signature on Windows.

src/DebugMessage.cpp

@@ -35,6 +35,9 @@ namespace Magnum {
 
 namespace {
 
+#ifdef CORRADE_TARGET_WINDOWS
+APIENTRY
+#endif
 void callbackWrapper(GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length, const GLchar* message, const void* userParam) {
     Context::current()->state().debug->messageCallback(DebugMessage::Source(source), DebugMessage::Type(type), id, DebugMessage::Severity(severity), std::string(message, length), userParam);
 }