Add common extension "m4a" for MPEG 4 Audio to

mime.types.

As a reference see Wikipedia:
https://en.wikipedia.org/wiki/MPEG-4_Part_14#.MP4_versus_.M4A

PR: 57895


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1723567 13f79535-47bb-0310-9956-ffa450edef68