[CMake] llvm_add_library: Add handling of the parameter ADDITIONAL_HEADERS to pass through to process_source.

I was insightless then about unknown optional parameters.
(Consider that LINK_LIBS foo bar ADDITIONAL_HEADERS qux quux)

Suggested by Michael Kruse. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201283 91177308-0d34-0410-b5e6-96231b3b80d8