Initial linear gradient 4f impl

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1688543002

Review URL: https://codereview.chromium.org/1688543002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 3a4d0e5..5444c1c 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 
+#include "Sk4fLinearGradient.h"
 #include "SkGradientShaderPriv.h"
 #include "SkLinearGradient.h"
 #include "SkRadialGradient.h"