Fixed build for openssl compiled with OPENSSL_NO_ENGINE.

Closes #2589. Thanks to Dirk Feytons.

ChangeLog.txt

@@ -55,6 +55,7 @@ Broker:
 - Fix mosquitto_plugin.h not including mosquitto_broker.h. Closes #2350.
 - Fix unlimited message quota not being properly checked for incoming
   messages. Closes #2593.
+- Fixed build for openssl compiled with OPENSSL_NO_ENGINE. Closes #2589.
 
 Client library:
 - Initialise sockpairR/W to invalid in `mosquitto_reinitialise()` to avoid

src/net.c

@@ -602,9 +602,11 @@ int net__tls_load_verify(struct mosquitto__listener *listener)
 	}
 #  endif
 
+#  if !defined(OPENSSL_NO_ENGINE)
 	if(net__load_engine(listener)){
 		return MOSQ_ERR_TLS;
 	}
+#  endif
 #endif
 	return net__load_certificates(listener);
 }