Return something in non-void function (#70)

Return NULL in `background_thread`, as
the return type is `gpointer`.

Fixes this error in a `-Wall -Werror` build:

    cpdb-frontend.c: In function 'background_thread':
    cpdb-frontend.c:465:1: error: control reaches end of non-void function [-Werror=return-type]
      465 | }
          | ^

cpdb/cpdb-frontend.c

@@ -461,6 +461,7 @@ gpointer background_thread(gpointer user_data) {
         if (f->stop_flag) break;
         cpdbActivateBackends(f);
     }
+    return NULL;
 }
 
 // Start the background thread