Replace MODULES_DIR with ASTMODDIR in Makefile's INSTALLDIRS

Post Asterisk 10, the MODULES_DIR variable no longer exists, and was replaced
with ASTMODDIR.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368855 65c4cc65-6c06-0410-ace0-fbb531ad65f3