arm/term: Fix linking error due multiple ps2_state definitions

When building with --target=arm-linux-gnu --with-platform=coreboot
a linking error occurs caused by multiple definitions of the
ps2_state variable.

Mark them as static since they aren't used outside their compilation unit.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/term/arm/cros.c

@@ -30,7 +30,7 @@
 #include <grub/fdtbus.h>
 #include <grub/arm/cros_ec.h>
 
-struct grub_ps2_state ps2_state;
+static struct grub_ps2_state ps2_state;
 
 struct grub_cros_ec_keyscan old_scan;