make display message one-time

It had an SDF specified, but I don't want to trust
that this SDF is safe to use considering it wasn't
a one-time node. So I picked a new, safe one.

rsrc/scenarios/stealth/scenario.xml

@@ -759,5 +759,6 @@
             <item chance="15">0</item>
         </storage>
         <sdf row="349" col="0">icy tunnels voice</sdf>
+        <sdf row="349" col="1">empire outpost library</sdf>
     </editor>
 </scenario>

rsrc/scenarios/stealth/towns/town15.spec

@@ -152,8 +152,8 @@
 	ex1 -1, -1, -1
 	ex2 -1, -1, -1
 	goto -1
-@disp-msg = 22
-	sdf 15, 3
+@once-disp-msg = 22
+	sdf 349, 1
 	msg 30, 31, -1
 	pic 0, 4
 	ex1 -1, -1, -1