[clang-tidy] Add dependency on clang-headers

Currently, to be able to process a source file including e.g. stddef.h with
clang-tidy, one has to build both clang-tidy and the clang-headers target.
Since stddef.h is needed for virtually any source file, let clang-tidy depend
on clang-headers, so that it Just Works after it has been built.

http://reviews.llvm.org/D22046

llvm-svn: 274751