vswitchd: Fix function prototype of packet_set_ipv6()

Follwoing patch fixes sparse error:
lib/packets.c:643:1: error: symbol 'packet_set_ipv6' redeclared
with different type (originally declared at lib/packets.h:493)
- incompatible argument 6 (different base types)

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>