[libclang] Add dummy libclang-headers target

`LLVM_DISTRIBUTION_COMPONENTS` assumes that each component has both
`component` and `install-component` targets. Add a dummy no-op target
for `libclang-headers` to placate this check.

Differential Revision: https://reviews.llvm.org/D39524

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317188 91177308-0d34-0410-b5e6-96231b3b80d8