cmake: Fix installation of include files

Install the configured Config/config.h file, rather than the
Config/config.h.cmake and Config/Config.h.in source files.

Contributed-by: Richard Membarth <richard.membarth@informatik.uni-erlangen.de>

Merged from: https://llvm.org/svn/llvm-project/polly/trunk@168780

llvm-svn: 170741