fixed terrain shadows.

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardSimpleLighting.shader

             #pragma multi_compile _ _VERTEX_LIGHTS
             #pragma multi_compile _ _MIXED_LIGHTING_SUBTRACTIVE
             #pragma multi_compile _ FOG_LINEAR FOG_EXP2
+            #pragma multi_compile _ _SHADOWS_ENABLED
-            #pragma multi_compile _ _SHADOWS_ENABLED
 
             //--------------------------------------
             // GPU Instancing

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardTerrain.shader

             #pragma multi_compile _ _VERTEX_LIGHTS
             #pragma multi_compile _ _MIXED_LIGHTING_SUBTRACTIVE
             #pragma multi_compile _ FOG_LINEAR FOG_EXP2
+            #pragma multi_compile _ _SHADOWS_ENABLED
 
             // -------------------------------------
             // Unity defined keywords