Add post_regtest_checks check_headers_and_includes to dist_noinst_SCRIPTS.

Also allow a .git directory in check_headers_and_includes to help
people using a git mirror of valgrind/VEX svn.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13601 a5019735-40e9-0310-863c-91ae7b9d1cf9