Config.in.legacy: select BR2_LEGACY from _XF86_INPUT_KEYBOARD

Commit 1100ead7b318 (package/x11r7/xdriver_xf86-input-keyboard: remove
package) added a legacy symbol for BR2_PACKAGE_XDRIVER_XF86_INPUT_KEYBOARD,
but forgot to select BR2_LEGACY.  Fix that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in.legacy

@@ -210,6 +210,7 @@ config BR2_PACKAGE_QEMU_CUSTOM_TARGETS_WRAP
 
 config BR2_PACKAGE_XDRIVER_XF86_INPUT_KEYBOARD
 	bool "xf86-input-keyboard removed"
+	select BR2_LEGACY
 	help
 	  The X.org keyboard input driver no longer support Linux.