add SK_API to SkShaderBase

Ought to fix Chrome roll:

    warning C4275: non dll-interface class 'SkShaderBase' used as base for
    dll-interface class 'SkEmptyShader'

etc.  May fix Android too?

Change-Id: Ic8ff988b9000c484aa54959e49a1ff17e5635340
Reviewed-on: https://skia-review.googlesource.com/18025
Reviewed-by: Florin Malita <fmalita@chromium.org>

src/core/SkShaderBase.h

@@ -22,7 +22,7 @@ struct SkImageInfo;
 class SkPaint;
 class SkRasterPipeline;
 
-class SkShaderBase : public SkShader {
+class SK_API SkShaderBase : public SkShader {
 public:
     SkShaderBase(const SkMatrix* localMatrix = nullptr);