Don't force OBJS_DEPEND_GUESS headers onto all objects.

This is in the case of not having any .depend.foo.o yet. Don't force add *.h
as a dependency for those. They are built in beforebuild already when in
SRCS/DPSRCS.

This change allows custom rules, like in bin/sh/Makefile for mksyntax, to not
have cyclic dependency problems when connected to the .depend.* handling.

This is purposely not copied to sys/conf/kern.post.mk as it handles
generating headers slightly differently.

MFC after: 2 weeks
Sponsored by: DellEMC