monitor/qmp-cmds.c: Don't include ui/vnc.h

The qmp-cmds.c file currently includes ui/vnc.h, which (being located
in the ui/ directory rather than include) is really supposed to be
for use only by the ui subsystem.  In fact the function prototypes we
need (vnc_display_password(), etc) are all declared in
include/ui/console.h, so we can switch to including that instead.

(ui/vnc.h includes include/ui/console.h, so this change strictly
reduces the quantity of headers qmp-cmds.c pulls in.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210104161200.15068-1-peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>