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: NAntony Pavlov <antonynpavlov@gmail.com>
Reviewed-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

vl.c

@@ -4267,6 +4267,7 @@ int main(int argc, char **argv, char **envp)
     /* init local displays */
     switch (display_type) {
     case DT_NOGRAPHIC:
+        (void)ds;	/* avoid warning if no display is configured */
         break;
 #if defined(CONFIG_CURSES)
     case DT_CURSES: