greybus: loopback: fix build breakage about SZ_4K

x86 doesn't include SZ_4K somehow so explicitly include <linux/sizes.h>
to fix the build breakage.
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/loopback.c

@@ -12,6 +12,7 @@
 #include <linux/kthread.h>
 #include <linux/delay.h>
 #include <linux/random.h>
+#include <linux/sizes.h>
 #include "greybus.h"
 
 struct gb_loopback_stats {