[nrf fromtree] net: events: Add missing net_mgmt.h include

net_event.h header makes use of macros defined in net_mgmt.h, therefore
it should include that header.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit ae7d7a800e281b42068a56cda3a7c5be56639600)

Author: rlubos

include/zephyr/net/net_event.h

@@ -13,6 +13,7 @@
 #define ZEPHYR_INCLUDE_NET_NET_EVENT_H_
 
 #include <zephyr/net/net_ip.h>
+#include <zephyr/net/net_mgmt.h>
 #include <zephyr/net/hostname.h>
 
 #ifdef __cplusplus