ccan/endian.h: Only define __{BIG,LITTLE}_ENDIAN if undefined.

ccan/endian.h: Only define __{BIG,LITTLE}_ENDIAN if undefined.

When both endian.h and ccan/endian/endian.h are included, we can have
__{BIG,LITTLE}_ENDIAN redefined when compiling with clang on FreeBSD.
Clang and gcc have moved to a predefine for endian orders. glibc defines
these the same as they are defied here, but that's an unsafe assumption
to make. Instead, only define them when __LITTLE_ENDIAN not defined as a
fallback to when the host does not define them in the standard system
headers.

Signed-off-by: Warner Losh <imp@bsdimp.com>