gpg: Fix "Too many open files" when using --multifile.

* common/miscellaneous.c (is_file_compressed): Don't cache the file.

This seems to be a pretty old bug. The fix is easy and also reveals
that -z0 can be used as a workaround.

• GnuPG-bug-id: T3951
• Signed-off-by: Werner Koch <wk@gnupg.org>