HID: egalax: update slab.h include

Implicit slab.h inclusion via percpu.h is about to go away.  Make sure
slab.h is included as necessary.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-egalax.c

@@ -16,6 +16,7 @@
 #include <linux/hid.h>
 #include <linux/module.h>
 #include <linux/usb.h>
+#include <linux/slab.h>
 #include "usbhid/usbhid.h"
 
 MODULE_AUTHOR("Stephane Chatty <chatty@enac.fr>");