pressure-vessel: Fix build regression with GLib 2.80.x

With older GLib, libglnx.h was enough to provide g_fdwalk_set_cloexec(), but with newer GLib, glib-unix.h is required.

Origin: https://github.com/flatpak/flatpak/pull/5737
Fixes: 7fac9ccb "pv-wrap: Use g_fdwalk_set_cloexec() instead of flatpak_close_fds_workaround()"