add missing const

If you want to declare a pointer which the address pointed to is constant,
you should declare it as like `static const char *const var = "...";`.

This commit supplies missing `const` and prevents some programming
error in the future.

src/eat.c

@@ -2198,7 +2198,7 @@ eatspecial(void)
 
 /* NOTE: the order of these words exactly corresponds to the
    order of oc_material values #define'd in objclass.h. */
-static const char *foodwords[] = {
+static const char *const foodwords[] = {
     "meal",    "liquid",  "wax",       "food", "meat",     "paper",
     "cloth",   "leather", "wood",      "bone", "scale",    "metal",
     "metal",   "metal",   "silver",    "gold", "platinum", "mithril",