ipv6: move udp declarations to net/udp.h

Fix the following compilation warning:

net/ipv6/udp.c:1031:30: warning: no previous prototype for 'udp_v6_early_demux' [-Wmissing-prototypes]
 1031 | INDIRECT_CALLABLE_SCOPE void udp_v6_early_demux(struct sk_buff *skb)
      |                              ^~~~~~~~~~~~~~~~~~
net/ipv6/udp.c:1072:29: warning: no previous prototype for 'udpv6_rcv' [-Wmissing-prototypes]
 1072 | INDIRECT_CALLABLE_SCOPE int udpv6_rcv(struct sk_buff *skb)
      |                             ^~~~~~~~~

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>