hmp: Compile hmp_info_spice() only with CONFIG_SPICE

It's dead code when CONFIG_SPICE is off.  If it wasn't, it would crash
dereferencing the null pointer returned by the qmp_query_spice()
dummy in qmp.c.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
