Ignore .dir-locals.el

Because we recently encourage people to have a .dir-locals.el, it's a good
idea to ignore it on a git level.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit d7c02691a5)

.gitignore

@@ -7,6 +7,7 @@
 .#*
 #*#
 *~
+/.dir-locals.el
 
 # Top level excludes
 /Makefile.bak