urilauncher: Fix use-after-free on GCC < 12

Building GTK with GCC 8 results in the following warning:

  gtk/gtkurilauncher.c: In function ‘gtk_uri_launcher_launch’:
  gtk/gtkurilauncher.c:315:3: warning: this ‘else’ clause does not guard... [-Wmisleading-indentation]
     else
     ^~~~
  gtk/gtkurilauncher.c:317:1: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘else’
   G_GNUC_BEGIN_IGNORE_DEPRECATIONS
   ^~~

In the compiled code, gtk_show_uri_full () is invoked whether the portal
branch is taken or not, leading to use-after-free of the task.

It looks like GCC in versions older than 12 treats the _Pragma(s) that
G_GNUC_BEGIN_IGNORE_DEPRECATIONS expands to as C-level statements, and
therefore the pragma takes up the 'else' statement slot.

See https://godbolt.org/z/e5zqbaqxo for a simple reproducer.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>

gtk/gtkurilauncher.c

@@ -314,9 +314,11 @@ gtk_uri_launcher_launch (GtkUriLauncher      *self,
     gtk_openuri_portal_open_uri_async (self->uri, parent, cancellable, open_done, task);
   else
 #endif
+    {
 G_GNUC_BEGIN_IGNORE_DEPRECATIONS
-    gtk_show_uri_full (parent, self->uri, GDK_CURRENT_TIME, cancellable, show_uri_done, task);
+      gtk_show_uri_full (parent, self->uri, GDK_CURRENT_TIME, cancellable, show_uri_done, task);
 G_GNUC_END_IGNORE_DEPRECATIONS
+    }
 }
 
 /**