meson: don't check for libutil.h except on FreeBSD
meson: don't check for libutil.h except on FreeBSD

The libutil.h we are after is explicitly only something we want on
FreeBSD, we don't want to accidentally pick up this header on other
platforms as it can lead to build failures.

Reviewed-by: Neal Gompa <8135daa3762340227c0c67f1c47ad07a127bd3a3@gmail.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,libvirt/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt