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>

package/qemu/Config.in

@@ -118,7 +118,9 @@ comment "Frontends"
 
 config BR2_PACKAGE_QEMU_SDL
 	bool "Enable SDL frontend"
+	depends on BR2_PACKAGE_XORG7
 	select BR2_PACKAGE_SDL
+	select BR2_PACKAGE_SDL_X11
 	help
 	  Say 'y' to enable the SDL frontend, that is, a graphical
 	  window presenting the VM's display.