build: get dependency file directories from object file names

After commit dcff25f2, Dependency file
are taken from the directories that have a Makefile.objs file.  This is
not enough, since files can be included from other directories.
So, pick them from directories that have an object file in them.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>