Syntax clean up to make clang happy.

Change-Id: I0984c395bbd1ee4b206eafd19915b6f68781dd16
diff --git a/src/compiler.cc b/src/compiler.cc
index c34b7d8..b06f718 100644
--- a/src/compiler.cc
+++ b/src/compiler.cc
@@ -1003,7 +1003,7 @@
   const CompilationContext* const context_;
   const size_t begin_;
   const size_t end_;
-  const Callback* callback_;
+  Callback* callback_;
   const size_t stripe_;
 
   friend void ForAll(CompilationContext*, size_t, size_t, Callback, size_t);