[FR] cc_libraries should use "includes", not "strip_include_prefix"

So that when users import google-benchmark, the headers are included as "system headers", instead of regular headers.

If this is not done, compilers will throw warnings at google-benchmark code, which users have no way to act upon. Users should be able to choose a highly restrictive set of warnings without having to fix them in google-benchmark.

Note that this pattern is already applied in the googletest repo.