include/denc: add missing {encode,decode}_nohead for denc_traits

include/denc: add missing {encode,decode}_nohead for denc_traits

implement {encode,decode}_nohead() for dec_traits<basic_string> and
bufferlist.

so compiler can find an appropriate version of decode_nohead() when
performing overload resolution. in some architectures, compiler resolves
the __u32 to size_t, which leads to a compilation failure, because the
compiler fails to instantiate the ::decode_nohead() template exposing
traits::decode_nohead().

Fixes: http://tracker.ceph.com/issues/18938
Signed-off-by: Kefu Chai <kchai@redhat.com>