maint: ignore transient files during tests

I ran 'git add .' for a patch in progress, while in the middle
of running 'make check' to test my work, and was surprised when
it picked up some files I wasn't expecting.

* .gitignore: Ignore *.pem.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gitignore

@@ -13,6 +13,7 @@
 *.loT
 *.o
 *.orig
+*.pem
 *.pyc
 *.rej
 *.s