.gitignore: "git-verify-commit" is a generated file

builtin/verify-commit.c was added in commit d07b00b ("verify-commit:
scriptable commit signature verification", 2014-06-23), update
.gitignore to ignore the generated file.

Signed-off-by: Øyvind A. Holm <sunny@sunbase.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>