loader: Move definition of ast_module_reload from _private.h to module.h

loader: Move definition of ast_module_reload from _private.h to module.h

No functionality change.  Just move the definition of ast_module_reload
from _private.h to module.h so it can be public.

Also removed the include of _private.h from manager.c since ast_module_load
was the only reason for including it.

Tested-by: George Joseph
Review: https://reviewboard.asterisk.org/r/4251/
........

Merged revisions 429542 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@429543 65c4cc65-6c06-0410-ace0-fbb531ad65f3