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: NRich Salz <rsalz@openssl.org>

.gitignore

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