Recursively search for '.c' files when making documentation with 'make full'

Without this, documentation defined in sub-folders is ignored. Since having
properly generated documentation is especially important in Asterisk 12 -
not having it can cause a module to not load - 'make full' needs to look in
all .c files.
........

Merged revisions 397924 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397925 65c4cc65-6c06-0410-ace0-fbb531ad65f3

Makefile

@@ -495,7 +495,7 @@ else
 	@echo "<docs xmlns:xi=\"http://www.w3.org/2001/XInclude\">" >> $@
 	@for x in $(MOD_SUBDIRS); do \
 		printf "$$x " ; \
-		for i in $$x/*.c; do \
+		for i in `find $$x -name '*.c'`; do \
 			$(PYTHON) build_tools/get_documentation.py < $$i >> $@ ; \
 		done ; \
 	done