dcb: fix sparse warnings

Add header with function definitions to quiet warnings and avoid future errors.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dcb/dcbevent.c

@@ -20,6 +20,7 @@
 #include <linux/rtnetlink.h>
 #include <linux/notifier.h>
 #include <linux/export.h>
+#include <net/dcbevent.h>
 
 static ATOMIC_NOTIFIER_HEAD(dcbevent_notif_chain);