rpc: export xdr_virNetMessageError for virnetmessagetest on Win32

rpc: export xdr_virNetMessageError for virnetmessagetest on Win32

The Win32 symbol export file format can't do wildcards, so none of
the 'xdr_*' symbols are exported from the libvirt DLL. This doesn't
matter generally since the RPC client is built into the DLL and we
don't build libvirtd on Win32. The virnetmessagetest, however, does
require xdr_virNetMessageError to be exported, so just do a hack for
that.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>