Fixed warning.

ScriptableRenderPipeline/LightweightPipeline/LWRP/Editor/ShaderPreprocessor.cs

 {
     public class ShaderPreprocessor : IPreprocessShaders
     {
+#if LOG_VARIANTS
+#endif
 
         // Multiple callback may be implemented.
         // The first one executed is the one where callbackOrder is returning the smallest number.