EmojiCompletion: Avoid un/signed compare warnings

(A) Use gsize to match the result of g_variant_n_children
(B) Use guint for n_matches, like the struct (and all other n_matches)

gtk/gtkemojicompletion.c

@@ -407,7 +407,7 @@ static gboolean
 has_variations (GVariant *emoji_data)
 {
   GVariant *codes;
-  int i;
+  gsize i;
   gboolean has_variations;
 
   has_variations = FALSE;
@@ -555,8 +555,8 @@ populate_completion (GtkEmojiCompletion *completion,
                      guint                offset)
 {
   GList *children, *l;
-  int n_matches;
-  int n_added;
+  guint n_matches;
+  guint n_added;
   GVariantIter iter;
   GVariant *item;