Fix cyclic dependency after r326552.

The OBJS_DEPEND_GUESS mechanism was making vmx_genassym.o depend
on all headers along with vmx_assym.h, though vmx_assym.h depends
on having vmx_genassym.o present to generate. Moving the headers
to DPSRCS is enough to resolve the issue as they will no longer
be implicit dependencies for all objects. Because of this we
need explicit OBJS_DEPEND_GUESS entries to ensure the headers
are generated when needed for the *_support.o files that need
them.

X-MFC-With: r326552
MFC after: 2 weeks
Sponsored by: Dell EMC