ci: Ignore files in meson-private in scan-build runs

These are typically files generated by the gobject-introspection dumper,
which we don’t want to scan as they are not part of GLib’s runtime code.

For example:
```
/builds/GNOME/glib/_scan_build/meson-private/tmpr3jwvyib/tmp-introspect5dmnb_je/GObject-2.0.c:799:27: warning: Access to field 'message' results in a dereference of a null pointer (loaded from variable 'error') [core.NullDereference]
  799 |       g_printerr ("%s\n", error->message);
      |                           ^~~~~~~~~~~~~~
1 warning generated.
```

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

.gitlab-ci.yml

@@ -787,6 +787,7 @@ scan-build:
       --exclude girepository/cmph/
       --exclude glib/libcharset/
       --exclude gio/xdgmime/
+      --exclude meson-private/
       -disable-checker deadcode.DeadStores
       --status-bugs
   before_script: