fix a 'multiple definition' error when included by multiple sources:
src/common/versioned_variant.h:31: multiple definition of `ceph::encode(std::monostate const&, ceph::buffer::v15_2_0::list&)';
rgw_main.cc.o:src/common/versioned_variant.h:31: first defined here
Signed-off-by: Casey Bodley <cbodley@redhat.com>