meson: remove obsolete check for LOOP_CTL_GET_FREE

meson: remove obsolete check for LOOP_CTL_GET_FREE

The LOOP_CTL_GET_FREE constant was introduced to Linux in

  commit 770fe30a46a12b6fb6b63fbe1737654d28e84844
  Author: Kay Sievers <kay.sievers@vrfy.org>
  Date:   Sun Jul 31 22:08:04 2011 +0200

    loop: add management interface for on-demand device allocation

This is old enough that all our supported platforms can be assumed
to have this feature. As a plus point, this meson check is going
to start failing with future GCC. It fails to set _GNU_SOURCE, thus
'unshare' is not defined by the header, and its relying on an
implicit function decl. For added fun this whole meson check was
semantically insane because LOOP_CTL_GET_FREE is not a valid arg
to unshare().

Fixes https://fedoraproject.org/wiki/Toolchain/PortingToModernC
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>