make rat silo message a oneshot. Fix #713

Author: NQNStudios

rsrc/scenarios/valleydy/scenario.xml

@@ -492,5 +492,6 @@
             <item chance="8">14</item>
             <item chance="10">16</item>
         </storage>
+        <sdf row="349" col="0">sweetgrove rat silo</sdf>
     </editor>
 </scenario>

rsrc/scenarios/valleydy/towns/town1.spec

@@ -1,5 +1,5 @@
-@disp-msg = 0
-	sdf -1, -1
+@once-disp-msg = 0
+	sdf 349, 0
 	msg 0, -1, -1
 	pic 0, 4
 	ex1 -1, -1, -1