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: NAlexander Graf <agraf@suse.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

include/net/checksum.h

@@ -19,6 +19,7 @@
 #define QEMU_NET_CHECKSUM_H
 
 #include <stdint.h>
+struct iovec;
 
 uint32_t net_checksum_add_cont(int len, uint8_t *buf, int seq);
 uint16_t net_checksum_finish(uint32_t sum);