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: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NGerd Hoffmann <kraxel@redhat.com>

hmp.c

@@ -535,6 +535,7 @@ out:
     qapi_free_VncInfo(info);
 }
 
+#ifdef CONFIG_SPICE
 void hmp_info_spice(Monitor *mon, const QDict *qdict)
 {
     SpiceChannelList *chan;
@@ -581,6 +582,7 @@ void hmp_info_spice(Monitor *mon, const QDict *qdict)
 out:
     qapi_free_SpiceInfo(info);
 }
+#endif
 
 void hmp_info_balloon(Monitor *mon, const QDict *qdict)
 {