Fix the Windows build (broken in 315a8ba)

Source/Core/VideoBackends/D3D/TextureCache.cpp

@@ -32,7 +32,7 @@ void TextureCache::TCacheEntry::Bind(unsigned int stage)
 	D3D::context->PSSetShaderResources(stage, 1, &texture->GetSRV());
 }
 
-bool TextureCache::TCacheEntry::Save(const std::string filename, unsigned int level)
+bool TextureCache::TCacheEntry::Save(const std::string& filename, unsigned int level)
 {
 	// TODO: Somehow implement this (D3DX11 doesn't support dumping individual LODs)
 	static bool warn_once = true;

Source/Core/VideoBackends/D3D/TextureCache.h

@@ -35,7 +35,7 @@ private:
 			const float *colmat);
 
 		void Bind(unsigned int stage);
-		bool Save(const std::string filename, unsigned int level);
+		bool Save(const std::string& filename, unsigned int level);
 	};
 
 	TCacheEntryBase* CreateTexture(unsigned int width, unsigned int height,