vl: fix build when configured with no graphic support

vl: fix build when configured with no graphic support

The following error occurs when building with no graphic output support:

  vl.c: In function ‘main’:
  vl.c:2829:19: error: variable ‘ds’ set but not used [-Werror=unused-but-set-variable]
       DisplayState *ds;
                     ^
  cc1: all warnings being treated as errors

To reproduce this issue, just run:

  $ ./configure \
      --disable-curses \
      --disable-sdl \
      --disable-cocoa \
      --disable-gtk \
      --disable-vnc \
      --enable-werror
  $ make vl.o

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>