Futureproofing hypothetical type mismatches

If we ever want huge maps with COLNO or ROWNO larger than signed char,
this will at least allow the game to compile and start when typedef'ing
xchar to int. Trying to use huge maps exposes more bugs.

include/mkroom.h

@@ -9,7 +9,7 @@
 /* mkroom.h - types and structures for room and shop initialization */
 
 struct mkroom {
-    schar lx, hx, ly, hy; /* usually xchar, but hx may be -1 */
+    xchar lx, hx, ly, hy; /* usually xchar, but hx may be -1 */
     schar rtype;          /* type of room (zoo, throne, etc...) */
     schar orig_rtype;     /* same as rtype, but not zeroed later */
     schar rlit;           /* is the room lit ? */