Revert "flatpak-utils-http: Don’t use GMainLoop"
Revert "flatpak-utils-http: Don’t use GMainLoop"

This reverts commit c1561b1dedfbf98b2efb842ee66165421ab1422b,
which seems to trigger a hang in the
test-oci-registry@system.wrap test-case.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak