stubs: Restrict ui/win32-kbd-hook to system-mode

stubs: Restrict ui/win32-kbd-hook to system-mode

In Makefile.objs, the ui/ directory is restricted to system-mode:

 43 ifeq ($(CONFIG_SOFTMMU),y)
 ...
 65 common-obj-y += ui/
 66 common-obj-m += ui/
 ...
 82 endif # CONFIG_SOFTMMU

Restrict the ui/ stub added in commit 2df9f5718df to only build
it for system-mode emulation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200522172510.25784-14-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>