nfs: add missing linux/types.h

After merging the nfs tree, today's linux-next build (powerpc allyesconfig
produced this warning:

./usr/include/linux/nfs.h:40: found __[us]{8,16,32,64} type without #include <linux/types.h>

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>