rhashtable: Fix undeclared EEXIST build error on ia64

We need to include linux/errno.h in rhashtable.h since it doesn't
always get included otherwise.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>