Fix a compile problem on FreeBSD for a missing header.

In this specific case the problem triggered on app_amd.c,
but it keeps coming out from time to time so it is better
to fix it in a more central place.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@14320 65c4cc65-6c06-0410-ace0-fbb531ad65f3