Fix macro conflict in NetBSD
* cipher/bithelp.h (bswap32): Rename to _gcry_bswap32. (bswap64): Rename to _gcry_bswap64.
NetBSD provides system macros bswap32 and bswap64 which conflicts with
our macros. Prefixing them with _gcry_ is easier than to come up with
a proper test.
(cherry picked from commit 36214bfa8f612cd2faa4de217d1a12a8b5faadbf)
- GnuPG-bug-id: T1600
- Signed-off-by: Werner Koch <email@example.com>