lib: fix byte order test in libsha1.c

lib: fix byte order test in libsha1.c

Previously PLATFORM_BYTE_ORDER and IS_LITTLE_ENDIAN were not defined,
so the little endian code was always compiled in.

This will have the effect that the "SHA1s" on big endian architectures
will change (i.e. become actual sha1s). So someone re-indexing their
database could conceivable lose tags on messages without a message-id
header.