fix sksl standalone build error

Bug: skia:
Change-Id: I7de6d46a5774b5820bb6468a418be7c56da8510b
Reviewed-on: https://skia-review.googlesource.com/121421
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

src/sksl/SkSLUtil.h

@@ -170,6 +170,10 @@ public:
     bool incompleteShortIntPrecision() const {
         return false;
     }
+
+    const char* fbFetchColorName() const {
+        return nullptr;
+    }
 };
 
 extern StandaloneShaderCaps standaloneCaps;