Adding clang-headers as dependency of cpp11-migrate

Fix build dependency. Now can say 'make cpp11-migrate' (with cmake anyway) from
a clean build and result will be usable.

Author: Ariel J Bernal <ariel.j.bernal@intel.com>


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@177871 91177308-0d34-0410-b5e6-96231b3b80d8