Fix mosquitto_plugin.h not including mosquitto_broker.h.

Closes #2350. Thanks to Wolfgang Petroschka.

ChangeLog.txt

@@ -12,6 +12,7 @@ Broker:
 - Fix client id not showing in log on failed connections, where possible.
 - Fix broker sending duplicate CONNACK on failed MQTT v5 reauthentication.
   Closes #2339.
+- Fix mosquitto_plugin.h not including mosquitto_broker.h. Closes #2350.
 
 Client library:
 - Initialise sockpairR/W to invalid in `mosquitto_reinitialise()` to avoid

include/mosquitto_plugin.h

@@ -44,6 +44,8 @@ extern "C" {
 #include <stdbool.h>
 #include <stdint.h>
 
+#include <mosquitto_broker.h>
+
 struct mosquitto;
 
 struct mosquitto_opt {