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

builtin/verify-commit.c was added in commit d07b00b7 ("verify-commit:
scriptable commit signature verification", 2014-06-23), update
.gitignore to ignore the generated file.
Signed-off-by: NØyvind A. Holm <sunny@sunbase.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.gitignore

@@ -165,6 +165,7 @@
 /git-upload-archive
 /git-upload-pack
 /git-var
+/git-verify-commit
 /git-verify-pack
 /git-verify-tag
 /git-web--browse