move struct propname to a header file

In doing so, avoid repeated layout declaration in cmd.c

src/cmd.c

@@ -1952,10 +1952,7 @@ static int
 wiz_intrinsic(void)
 {
     if (wizard) {
-        extern const struct propname {
-            int prop_num;
-            const char *prop_name;
-        } propertynames[]; /* timeout.c */
+        extern const struct propname propertynames[]; /* timeout.c */
         static const char wizintrinsic[] = "#wizintrinsic";
         static const char fmt[] = "You are%s %s.";
         winid win;