Makefile: fix dependencies for generated .h, .c

Makefile: fix dependencies for generated .h, .c

We have a single rule generating .c and .h files,
so .h doesn't depend on .c: both depend on the
source schema.

Fix Makefile to reflect that - without this,
if .c is there and .h is missing, Makefile does
not know how to remake .h and assumes it's
a dummy target, triggering endless rebuilds.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>