compat/bswap.h: fix endianness detection

The changes to make detection of endianness more portable had a bug
that breaks on (at least) Solaris x86.

The bug appears to be a simple copy/paste typo. It checks for
_BIG_ENDIAN and not _LITTLE_ENDIAN for both the case where we would
decide the system is big endian and little endian. Instead, the
second test should be for _LITTLE_ENDIAN and not _BIG_ENDIAN.

Two fixes were possible:

 1. Change the negation order of the conditions in the second test.
 2. Reverse the order of the conditions in the second test.

Use the second option so that the condition we expect is always a
positive check.

Signed-off-by: Ben Walton <bdwalton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>