VideoCommon: fix compilation error in pixel shaders when per-pixel lighting isn't set for custom shaders

Source/Core/VideoCommon/PixelShaderGen.cpp

@@ -839,7 +839,8 @@ void WriteCustomShaderStructImpl(ShaderCode* out, u32 num_stages, bool per_pixel
                texcoord);
   }
 
-  GenerateCustomLightingImplementation(out, uid_data->lighting, "colors_");
+  if (per_pixel_lighting)
+    GenerateCustomLightingImplementation(out, uid_data->lighting, "colors_");
 
   for (u32 i = 0; i < 16; i++)
   {