doc-rst: Fix recursive make invocation from macros

In any case where we recurse but don't mention $(MAKE) literally in
the recipe, we need to add a '+' at the start of the command to ensure
that parallel makes and various other options work properly.

Fixes: 609afe6b

 ("Documentation/sphinx: build the media intermediate ...")
Tested-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>