Meson: Fix build of gtkdoc

This requires change added in Meson 0.44.0

https://bugzilla.gnome.org/show_bug.cgi?id=786796

docs/reference/gio/meson.build

@@ -52,9 +52,12 @@ if get_option('with-docs') != 'no'
     'gnetworkmonitorbase.h',
     'gnetworkmonitornetlink.h',
     'gnetworkmonitornm.h',
+    'gnetworkmonitorportal.h',
     'gnotificationbackend.h',
     'gnotification-private.h',
+    'gosxappinfo.h',
     'gpollfilemonitor.h',
+    'gproxyresolverportal.h',
     'gregistrysettingsbackend.h',
     'gresourcefile.h',
     'gsettingsbackendinternal.h',