net: declare struct iovec in checksum.h to fix compiler warning

The checksum calculation header exports a function that refers to
struct iov defined in iov.h.  Without including the former, build
fails like this:

  In file included from hw/net/fsl_etsec/rings.c:24:0:
  include/net/checksum.h:51:31: error: ‘struct iovec’ declared inside parameter list [-Werror]
  include/net/checksum.h:51:31: error: its scope is only this definition or declaration, which is probably not what you want [-Werror]

Mention struct iovec there.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
