Comment from Makefile.objs:

The system emulation needs this dependency (which was missing in Makefile),
otherwise builds without tools (or massive parallel builds) fail.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>