Ignore harmless patch backups

When patching `ltmain.sh` the `patch` utility creates this file if the
patch did not apply cleanly, e.g. when it applied with an offset.  Any
hunks that *failed* to apply end up in `ltmain.sh.rej`.  This file you
probably want to see in `git status` so it is not ignored.
It should not be committed, of course.

.gitignore

@@ -28,6 +28,7 @@ Makefile.in
 /depcomp
 /install-sh
 /ltmain.sh
+/ltmain.sh.orig
 /m4/
 /missing
 /mkinstalldirs