Include multibyte.h unconditionally.

Include multibyte.h unconditionally.
(VALID_CHARPTR_P): Moved to multibyte.h.
(ASSERT_VALID_CHARPTR): Likewise.
(REAL_INC_CHARPTR): Likewise.
(REAL_INC_CHARBYTIND): Likewise.
(REAL_DEC_CHARPTR): Likewise.
(INC_CHARPTR): Likewise.
(INC_CHARBYTIND): Likewise.
(DEC_CHARPTR): Likewise.
(VALIDATE_CHARPTR_BACKWARD): Likewise.
(VALIDATE_CHARPTR_FORWARD): Likewise.
(charptr_n_addr): Likewise.
(MAX_EMCHAR_LEN): Moved to mb-*.h.
(simple_charptr_emchar): Moved to multibyte.h.
(simple_set_charptr_emchar): Likewise.
(simple_charptr_copy_char): Likewise.
(non_ascii_charptr_emchar): Likewise.
(non_ascii_set_charptr_emchar): Likewise.
(non_ascii_charptr_copy_char): Likewise.
(charptr_emchar): Likewise.
(set_charptr_emchar): Likewise.
(charptr_copy_char): Likewise.
(charptr_emchar_n): Likewise.
(Charset_ID): Moved to char-1byte.h.
(Vcharset_ascii): Likewise.
(CHAR_CHARSET): Likewise.
(CHAR_LEADING_BYTE): Likewise.
(LEADING_BYTE_ASCII): Likewise.
(NUM_LEADING_BYTES): Likewise.
(MIN_LEADING_BYTE): Likewise.
(CHARSETP): Likewise.
(CHARSET_BY_LEADING_BYTE): Likewise.
(XCHARSET_LEADING_BYTE): Likewise.
(XCHARSET_GRAPHIC): Likewise.
(XCHARSET_COLUMNS): Likewise.
(XCHARSET_DIMENSION): Likewise.
(BREAKUP_CHAR): Likewise.