sksl: Add builtinFMASupport to StandaloneShaderCaps

Bug: skia:
Change-Id: I8c32b09d3c4042707c4d6fc957c7bbff41022ce1
Reviewed-on: https://skia-review.googlesource.com/c/171524
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>

src/sksl/SkSLUtil.h

@@ -137,6 +137,10 @@ public:
         return false;
     }
 
+    bool builtinFMASupport() const {
+        return true;
+    }
+
     const char* shaderDerivativeExtensionString() const {
         return nullptr;
     }