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
GNOME/glib#1495.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
8 jobs for gresource-distcheck-failure in 20 minutes and 58 seconds (queued for 20 seconds)
Status Job ID Name Coverage
  Build
passed #88682
cross-android_api21_arm64

00:01:54

passed #88683
cross-android_api28_arm64

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passed #88684
cross-mingw64

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passed #88681
fedora-x86_64

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manual #88687
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #88685
win32
msys2-mingw32

00:08:30

passed #88686
win32
vs2017-x64

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  Coverage
passed #88688
coverage

00:01:41

73.3%