Make sure various functions return something. Silences warnings on gcc 4.4

Patch by lukas at double dot net

src/dvb/dvb_tables.c

@@ -150,6 +150,7 @@ dvb_table_input(void *aux)
       }
     }
   }
+  return NULL;
 }
 
 

src/serviceprobe.c

@@ -167,6 +167,7 @@ serviceprobe_thread(void *aux)
     subscription_unsubscribe(s);
     was_doing_work = 1;
   }
+  return NULL;
 }
 
 

src/subscriptions.c

@@ -239,6 +239,7 @@ subscription_janitor(void *aux)
 
     pthread_mutex_unlock(&global_lock);
   }
+  return NULL;
 }
 
 

src/tcp.c

@@ -445,6 +445,7 @@ tcp_server_loop(void *aux)
       }
     }
   }
+  return NULL;
 }
 
 /**

src/xmltv.c

@@ -506,6 +506,7 @@ xmltv_thread(void *aux)
       free(p);
     }
   }
+  return NULL;
 }