qemu: sdl frontend needs x11

qemu: sdl frontend needs x11

Since qemu 2.12.0 and
https://github.com/qemu/qemu/commit/2ec78706d188df7d3dab43d07b19b05ef7800a44,
x_keymap.h has been converted from "SDL display driver" to "X11 keymaps"

So add a select on BR2_PACKAGE_SDL_X11

Fixes:
 - http://autobuild.buildroot.org/results/1908d2d7de8d3aff11ed6fbb8fe4cf3eff54b5a5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>