Move hexchar() from the sudoers plugin to lib/util.

plugins/sudoers/parse.h

@@ -417,9 +417,6 @@ struct gid_list *runas_getgroups(void);
 /* toke.c */
 void init_lexer(void);
 
-/* hexchar.c */
-int hexchar(const char *s);
-
 /* base64.c */
 size_t base64_decode(const char *str, unsigned char *dst, size_t dsize);
 size_t base64_encode(const unsigned char *in, size_t in_len, char *out, size_t out_len);