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: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NCorentin Chary <corentincj@iksaif.net>
Signed-off-by: NMatthew Garrett <mjg@redhat.com>

drivers/platform/x86/samsung-laptop.c

@@ -25,6 +25,7 @@
 #include <linux/acpi.h>
 #include <linux/seq_file.h>
 #include <linux/debugfs.h>
+#include <linux/ctype.h>
 
 /*
  * This driver is needed because a number of Samsung laptops do not hook