ipv6: ndisc: adjust ndisc_ifinfo_sysctl_change prototype

Commit 32927393 ("sysctl: pass kernel pointers to ->proc_handler")
changed ndisc_ifinfo_sysctl_change to take a kernel pointer. Adjust its
prototype in net/ndisc.h as well to fix the following sparse warning:

net/ipv6/ndisc.c:1838:5: error: symbol 'ndisc_ifinfo_sysctl_change' redeclared with different type (incompatible argument 3 (different address spaces)):
net/ipv6/ndisc.c:1838:5:    int extern [addressable] [signed] [toplevel] ndisc_ifinfo_sysctl_change( ... )
net/ipv6/ndisc.c: note: in included file (through include/net/ipv6.h):
./include/net/ndisc.h:496:5: note: previously declared as:
./include/net/ndisc.h:496:5:    int extern [addressable] [signed] [toplevel] ndisc_ifinfo_sysctl_change( ... )
net/ipv6/ndisc.c: note: in included file (through include/net/ip6_route.h):

Fixes: 32927393

 ("sysctl: pass kernel pointers to ->proc_handler")
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>