mime.types: Add common extension "m4a" for MPEG 4 Audio.

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

Submitted by: Dylan Millikin <dylan.millikin gmail.com>
PR: 57895
Backports: r1723567
Reviewed by: rjung, wrowe, ylavic



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1750837 13f79535-47bb-0310-9956-ffa450edef68