When $(MAKE) is expanded via a macro "make" doesn't know whether
$(MAKE) refers to an instance of the make app and therefore doesn't
pass job server arguments down.  This change adds the + prefix to the
command in the receipe in order to indicate $(MAKE) is another instance of
make.

Tested:
Verified flatc builds in parallel from Android builds.

Change-Id: I9f2f4f9680b818fdda7420a75a8bfa995b4644db