drivers, samsung-laptop: fix usage of isalnum

linux/ctype.h is needed for isalnum() to avoid a build error:

drivers/platform/x86/samsung-laptop.c: In function ‘samsung_sabi_diag’:
drivers/platform/x86/samsung-laptop.c:1306: error: implicit declaration of function ‘isalnum’

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>