net: unexport skb_gro_receive()

skb_gro_receive() used to be used by SCTP, it is no longer the case.

skb_gro_receive_list() is in the same category : never used from modules.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>