ipv6: fix incorrect type in declaration

Introduced by 1397ed35


  "ipv6: add flowinfo for tcp6 pkt_options for all cases"

Reported-by: kbuild test robot <fengguang.wu@intel.com>

V2: fix the title, add empty line after the declaration (Sergei Shtylyov
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Signed-off-by: David S. Miller <davem@davemloft.net>