bignum.c: suppress unused variable warning

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65749 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bignum.c

@@ -5089,7 +5089,9 @@ rb_big2str(VALUE x, int base)
 static unsigned long
 big2ulong(VALUE x, const char *type)
 {
+#if SIZEOF_LONG > SIZEOF_BDIGIT
     size_t i;
+#endif
     size_t len = BIGNUM_LEN(x);
     unsigned long num;
     BDIGIT *ds;