libmalcontent: Add missing field initialisers to BUILDER_INIT

This fixes use of libmalcontent in projects which build with
`-Wmissing-field-initializers`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

libmalcontent/app-filter.h

@@ -159,6 +159,9 @@ GType mct_app_filter_builder_get_type (void);
     g_hash_table_new_full (g_str_hash, g_str_equal, g_free, NULL), \
     TRUE, \
     FALSE, \
+    /* padding: */ \
+    NULL, \
+    NULL \
   }
 
 void mct_app_filter_builder_init  (MctAppFilterBuilder *builder);