libertas: add missing include

Without it, I get compile errors due to missing TASK_NORMAL,
TASK_UNINTERRUPTIBLE and schedule.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/libertas/firmware.c

@@ -5,6 +5,7 @@
 #include <linux/firmware.h>
 #include <linux/firmware.h>
 #include <linux/module.h>
+#include <linux/sched.h>
 
 #include "dev.h"
 #include "decl.h"