po/Makefile includes rules.mak to use the nice quiet-command macro.
However, this also brings in a %.mo rule that breaks "make build".
Put our own rule before the include, so that it has precedence.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>