vl: always define no_frame

vl: always define no_frame

Commit 047d4e151dd46 "Unbreak -no-quit for GTK, validate SDL options" broke
build of qemu without sdl, by referencing `no_frame' variable which is defined
inside #if SDL block.  Fix that by defining that variable unconditionally.

This is a better fix for the build issue introduced by that patch than
a revert.  This change keeps the new functinality introduced by that patch
and just fixes the compilation.  It still is not a complete fix around the
original issue (not working -no-frame et al with -display gtk), because it
makes only the legacy interface working, not the new suboption interface,
so a few more changes are needed.

Cc: Peter Wu <lekensteyn@gmail.com>
Cc: qemu-trivial@nongnu.org
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Wu <lekensteyn@gmail.com>