Don’t try to make includes/global.rst, it is included in the repo

And updated by humans. Done because apparently after
016d189 `make` on Travis CI does not
recognize anymore that the files are already up-to-date and remakes
them.

docs/Makefile

@@ -54,7 +54,7 @@ distclean:
 
 .PHONY: FORCE_MAKE
 
-prepare: includes/global.rst
+prepare:
 
 includes/global.rst: \
 		includes/30gen_ansible_modules.rst \