Added exp2 fog to low end shader

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardLowEnd.shader

             // -------------------------------------
             // Lightweight Pipeline keywords
             #pragma multi_compile _ _MIXED_LIGHTING_SUBTRACTIVE
-            #pragma multi_compile _ FOG_LINEAR
+            #pragma multi_compile _ FOG_LINEAR FOG_EXP2
 
             // -------------------------------------
             // Unity defined keywords