converted GrRectBlurEffect to SkSL

Bug: skia:
Change-Id: I3a8e16fd2792e6fb5711815d8aad46ae30c2872e
Reviewed-on: https://skia-review.googlesource.com/59163
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index 1de85f6..615b5dc 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -33,5 +33,6 @@
   "$_src/gpu/effects/GrConfigConversionEffect.fp",
   "$_src/gpu/effects/GrDitherEffect.fp",
   "$_src/gpu/effects/GrEllipseEffect.fp",
+  "$_src/gpu/effects/GrRectBlurEffect.fp",
   "$_src/gpu/effects/GrSimpleTextureEffect.fp",
 ]