Fix build error when VNC is configured out

Fix build error when VNC is configured out

In hmp_change(), the variable hmp_mon is only used
by code under #ifdef CONFIG_VNC. This results in a build
error when VNC is configured out with the default of
treating warnings as errors:

monitor/hmp-cmds.c: In function ‘hmp_change’:
monitor/hmp-cmds.c:1946:17: error: unused variable ‘hmp_mon’ [-Werror=unused-variable]
1946 |     MonitorHMP *hmp_mon = container_of(mon, MonitorHMP, common);
     |                 ^~~~~~~

Signed-off-by: Christophe de Dinechin <[email protected]>
Message-Id: <20190625123905[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>