build: Clean files left behind after gio/tests/gresource.c test

This is a follow-up to commit 614adf8a, which started generating two new files as part of the test; they need to be cleaned up before distcheck will pass.

Ideally, the test should run a temporary directory and wipe that directory itself before exiting, but that’s a bit of a big change to make right now. Deferred to #1495.

Signed-off-by: Philip Withnall withnall@endlessm.com