Removed unnecessary #undef ISPRINT in include/mruby.h

At line 302 in include/mruby.h we check if `ISPRINT` is not defined, then at line 305 we `#undef ISPRINT`

    /* need to include <ctype.h> to use these macros */
    #ifndef ISPRINT
    //#define ISASCII(c) isascii((int)(unsigned char)(c))
    #define ISASCII(c) 1
    #undef ISPRINT
    #define ISPRINT(c) (ISASCII(c) && isprint((int)(unsigned char)(c)))
    #define ISSPACE(c) (ISASCII(c) && isspace((int)(unsigned char)(c)))
    #define ISUPPER(c) (ISASCII(c) && isupper((int)(unsigned char)(c)))
    #define ISLOWER(c) (ISASCII(c) && islower((int)(unsigned char)(c)))
    #define ISALNUM(c) (ISASCII(c) && isalnum((int)(unsigned char)(c)))
    #define ISALPHA(c) (ISASCII(c) && isalpha((int)(unsigned char)(c)))
    #define ISDIGIT(c) (ISASCII(c) && isdigit((int)(unsigned char)(c)))
    #define ISXDIGIT(c) (ISASCII(c) && isxdigit((int)(unsigned char)(c)))
    #define TOUPPER(c) (ISASCII(c) ? toupper((int)(unsigned char)(c)) : (c))
    #define TOLOWER(c) (ISASCII(c) ? tolower((int)(unsigned char)(c)) : (c))
    #endif

Maybe I'm missing something but it sounds suspect to me: perhaps line 305 should be removed?