Add header file I accidentally removed in teh shuffle

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10795 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/ConstantProp.cpp

@@ -20,6 +20,7 @@
 
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Constant.h"
 #include "llvm/Instruction.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/InstIterator.h"