settings.gradle: add "dependencies" as maven repo

Otherwise the flatpak-gradle-generator plugin is not found in the
flatpak environment.

settings.gradle

@@ -1,6 +1,7 @@
 pluginManagement {
   repositories {
     maven { url="app/build/dependencies" }
+    maven { url="dependencies" }
     gradlePluginPortal()
   }
 }