test: Depend on a recent enough version of libportal

The notification API for the notification test is only in
current git (0.9.0). Without this the test fail to build if you have
the current release installed: this triggers the meson
subproject build.

Signed-off-by: Hubert Figuière <hub@figuiere.net>

meson.build

@@ -120,7 +120,7 @@ geoclue_dep = dependency(
 )
 libportal_dep = dependency(
   'libportal',
-  version: '>= 0.8.1',
+  version: '>= 0.9.0',
   required: get_option('libportal'),
   fallback: ['libportal', 'libportal_dep'],
   default_options: [