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>