Set a module load priority for format modules.

A recent change to app_voicemail made it such that the module now assumes that
all format modules are available while processing voicemail configuration.
However, when autoloading modules, it was possible that app_voicemail was
loaded before the format modules.  Since format modules don't depend on
anything, set a module load priority on them to ensure that they get loaded
first when autoloading.

This fix applies to trunk, 1.6.1, and 1.6.2.  The fix for 1.4 and 1.6.0 will
require a different approach since the module load priority functionality is
not present in the module API.

(issue #16412)
Reported by: jiddings


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@233692 65c4cc65-6c06-0410-ace0-fbb531ad65f3