net/core: move gro function declarations to separate header

Fir the following compilation warnings:
 1031 | INDIRECT_CALLABLE_SCOPE void udp_v6_early_demux(struct sk_buff *skb)

net/ipv6/ip6_offload.c:182:41: warning: no previous prototype for ‘ipv6_gro_receive’ [-Wmissing-prototypes]
  182 | INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
      |                                         ^~~~~~~~~~~~~~~~
net/ipv6/ip6_offload.c:320:29: warning: no previous prototype for ‘ipv6_gro_complete’ [-Wmissing-prototypes]
  320 | INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
      |                             ^~~~~~~~~~~~~~~~~
net/ipv6/ip6_offload.c:182:41: warning: no previous prototype for ‘ipv6_gro_receive’ [-Wmissing-prototypes]
  182 | INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
      |                                         ^~~~~~~~~~~~~~~~
net/ipv6/ip6_offload.c:320:29: warning: no previous prototype for ‘ipv6_gro_complete’ [-Wmissing-prototypes]
  320 | INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)

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