don't load bullet if module is not available

demo/demo/main.cpp

@@ -85,7 +85,9 @@ int main(int argc, const char* argv[]) {
 	moduleManager.use<fggl::display::GLFW>();
 	moduleManager.use<fggl::assets::AssetFolders>();
 	moduleManager.use<fggl::entity::ECS>();
-	moduleManager.use<fggl::phys::Bullet3>();
+	#ifdef FGGL_MODULE_BULLET
+		moduleManager.use<fggl::phys::Bullet3>();
+	#endif
 	moduleManager.resolve();
 
 	// create the application