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.

include/artilist.h

@@ -9,7 +9,7 @@
 #define A(nam, typ, s1, s2, mt, atk, dfn, cry, inv, al, cl, rac, \
           cost, clr, bn) nam
 
-static const char *artifact_names[] = {
+static const char *const artifact_names[] = {
 
 #elif defined(ARTI_ENUM)
 #define A(nam, typ, s1, s2, mt, atk, dfn, cry, inv, al, cl, rac, \