Both commit f1b3ccfa "monitor: Move {hmp, qmp}.c to monitor/{hmp,
qmp}-cmds.c" and commit 7e3c0dea "monitor: Split out monitor/qmp.c"
added monitor/ to common-obj-y ifeq ($(CONFIG_SOFTMMU),y).  Revert the
second addition.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190619201050.19040-3-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>