Added C++ guards.

Rev: src/nettle/aes.h:1.8
Rev: src/nettle/arcfour.h:1.6
Rev: src/nettle/arctwo.h:1.3
Rev: src/nettle/asn1.h:1.3
Rev: src/nettle/base16.h:1.4
Rev: src/nettle/base64.h:1.15
Rev: src/nettle/bignum.h:1.11
Rev: src/nettle/blowfish.h:1.10
Rev: src/nettle/buffer.h:1.6
Rev: src/nettle/cast128.h:1.6
Rev: src/nettle/cbc.h:1.9
Rev: src/nettle/ctr.h:1.3
Rev: src/nettle/des-compat.h:1.13
Rev: src/nettle/des.h:1.11
Rev: src/nettle/dsa.h:1.10
Rev: src/nettle/hmac.h:1.6
Rev: src/nettle/knuth-lfib.h:1.4
Rev: src/nettle/md2.h:1.4
Rev: src/nettle/md4.h:1.4
Rev: src/nettle/md5-compat.h:1.3
Rev: src/nettle/md5.h:1.9
Rev: src/nettle/memxor.h:1.5
Rev: src/nettle/nettle-meta.h:1.15
Rev: src/nettle/nettle-types.h:1.2
Rev: src/nettle/pgp.h:1.8
Rev: src/nettle/pkcs1.h:1.4
Rev: src/nettle/realloc.h:1.3
Rev: src/nettle/rsa-compat.h:1.4
Rev: src/nettle/rsa.h:1.26
Rev: src/nettle/serpent.h:1.8
Rev: src/nettle/sexp.h:1.18
Rev: src/nettle/sha.h:1.7
Rev: src/nettle/testsuite/testutils.h:1.24
Rev: src/nettle/twofish.h:1.7
Rev: src/nettle/yarrow.h:1.12