build: Automatically adding 'mobility' dependency if topology plugin is enabled

Otherwise, the build process was causing problems on FreeBSD
diff --git a/wscript b/wscript
index 9b61d52..e1e4bb6 100644
--- a/wscript
+++ b/wscript
@@ -64,6 +64,7 @@
 
     if 'topology' in bld.env['NDN_plugins']:
         deps.append ('topology-read')
+        deps.append ('mobility')
 
     if 'mobility' in bld.env['NDN_plugins']:
         deps.append ('mobility')