sandbox: enable USB_KEYBOARD_FN_KEYS

Enable the support of function keys on the USB keyboard. This is necessary
to test the USB keyboard driver.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 774c278..02969f9 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -163,6 +163,7 @@
 CONFIG_DM_USB=y
 CONFIG_USB_EMUL=y
 CONFIG_USB_KEYBOARD=y
+CONFIG_USB_KEYBOARD_FN_KEYS=y
 CONFIG_DM_VIDEO=y
 CONFIG_CONSOLE_ROTATION=y
 CONFIG_CONSOLE_TRUETYPE=y