Build System: Create Makefile macro MOD_ADD_SOURCE.

This new macro allows a single line to add all additional
sources to a module.  This helps prevent modules from
missing steps, and makes future changes easier since
they can be made in a single place.

ASTERISK-24960 #close
Reported by: Corey Farrell

Change-Id: I38f12d8b72c5e7bb37a879b2fb51761a2855eb4b