[Port] [6000.0] [2d] Fix Unlit shader for Skinning.

Author: venkify

Packages/com.unity.render-pipelines.universal/Shaders/2D/Sprite-Unlit-Default.shader

@@ -126,6 +126,7 @@ Shader "Universal Render Pipeline/2D/Sprite-Unlit-Default"
             #pragma vertex UnlitVertex
             #pragma fragment UnlitFragment
 
+            #pragma multi_compile _ SKINNED_SPRITE
             #pragma multi_compile_fragment _ DEBUG_DISPLAY
 
             struct Attributes