qemu: fix nativesdk build without x11

when x11 is not defined in DISTRO_FEATURES, there are no x11 headers
so True is not defined leading to :
| ui/sdl.c:62:8: warning: type defaults to 'int' in declaration of 'doing_grabs' [-Wimplicit-int]
|  static doing_grabs = True;
|         ^
| ui/sdl.c:62:22: error: 'True' undeclared here (not in a function)
|  static doing_grabs = True;
|                       ^

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>