compat/bswap: add include header guards

compat/bswap: add include header guards

Our compat/bswap.h lacks the usual preprocessor guards against multiple
inclusion. This usually isn't an issue since it only gets included from
git-compat-util.h, which has its own guards. But it would produce
redeclaration errors if any file included it separately.

Our hdr-check target would complain about this, except that it currently
skips items in compat/ entirely.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>