Fix following error:

netflowPlugin.c: In function 'netflowUtilsLoop':
netflowPlugin.c:2144:1: error: no return statement in function returning non-void [-Werror=return-type]
 2144 | }
      | ^

Return NULL like netflowMainLoop does.

--- ntop-5.0.1.orig/plugins/netflowPlugin.c	2012-03-09 17:44:17.000000000 +0000
+++ ntop-5.0.1/plugins/netflowPlugin.c	2019-08-09 10:25:40.878799852 +0000
@@ -2141,6 +2141,7 @@
       waitCondvar(&myGlobals.device[deviceId].netflowGlobals->ifStatsQueueCondvar);
     }
   }
+  return(NULL);
 }
 #endif