rpmsg: pull in slab.h

In preparation to virtio header changes, include slab.h directly as
this module is using it.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/rpmsg/mtk_rpmsg.c

@@ -8,6 +8,7 @@
 #include <linux/platform_device.h>
 #include <linux/remoteproc.h>
 #include <linux/rpmsg/mtk_rpmsg.h>
+#include <linux/slab.h>
 #include <linux/workqueue.h>
 
 #include "rpmsg_internal.h"