include/linux/byteorder: Fix compilation of __constant_cpu_to_be32()

The macro __constant_cpu_to_be32() uses ___constant_swab32(), which for
some reason is not defined and causes the following error during
compilation:

  include/linux/byteorder/little_endian.h:28:52: warning:
    implicit declaration of function ‘___constant_swab32’;
    did you mean ‘__builtin_bswap32’? [-Wimplicit-function-declaration]
   #define __constant_cpu_to_be32(x) ((__force __be32)___constant_swab32((x)))

Declare all ___constant_swabXX() macros.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>