Expose SkRuntimeShaderFactory class to clients

Test: Build and linked android
Change-Id: Ia98063fba2d2124335d71a5b1fe3a63f0c065f1e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257327
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Stan Iliev <stani@google.com>

src/shaders/SkRTShader.h

@@ -49,7 +49,7 @@ private:
     typedef SkShaderBase INHERITED;
 };
 
-class SkRuntimeShaderFactory {
+class SK_API SkRuntimeShaderFactory {
 public:
     SkRuntimeShaderFactory(SkString sksl, bool isOpaque);