jumper, hook in callback

Derp.  I added the code but never hooked it up.

Change-Id: I955fe36465809c891fdaf818bf1287843fbdf5fb
Reviewed-on: https://skia-review.googlesource.com/13598
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/jumper/SkJumper.cpp

@@ -41,6 +41,7 @@ static K kConstants = {
 };
 
 #define STAGES(M)         \
+    M(callback)           \
     M(seed_shader)        \
     M(constant_color)     \
     M(clear)              \