glibc: Avoid errors if you accidentally create a git symlink in the glibc source dir

glibc: Avoid errors if you accidentally create a git symlink in the glibc source dir

If you accidentally create symlinks in the glibc sources directory, it
fails in very hard to understand ways. Whilst most people don't do this,
since I debugged it, specifify the list of plugins we use to avoid
this biting anyone else.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>