Unhide monster when rolling boulder explodes

src/explode.c

@@ -842,6 +842,7 @@ scatter(int sx, int sy,  /* location of objects to scatter */
         (void) hideunder(&g.youmonst);
     if (((mtmp = m_at(sx, sy)) != 0) && mtmp->mtrapped)
         mtmp->mtrapped = 0;
+    maybe_unhide_at(sx, sy);
     if (lostgoods) /* implies shop_origin and therefore shkp valid */
         credit_report(shkp, 1, FALSE);
     return total;