Without this (at least in Fedora 35) it don't detect mremap()
correctly.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20220502131119.2345-1-quintela@redhat.com>
[Also switch the LEGACY_RDMA_REG_MR test to cc.links, otherwise
 Debian fails to build. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>