verify-tag: drop signal.h include

There's no reason verify-tag.c needs to include signal.h. It's already in git-compat-util.h, which we properly include as the first header. And there doesn't seem to be a particular reason for this include; it's just an artifact from the file creation in 2ae68fcb78 (Make verify-tag a builtin., 2007-07-27). Likewise verify-commit.c has the same issue, probably because it was created using verify-tag as a template in d07b00b7f3 (verify-commit: scriptable commit signature verification, 2014-06-23). These includes are probably just redundant, and not hurting anything by circumventing the order that git-compat-util.h tries to impose, since we'll always have loaded git-compat-util by the time we get to these. So this is just a cleanup, and shouldn't fix or break any platforms. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>