net: gro: move two declarations to include/net/gro.h

Move gro_find_receive_by_type() and gro_find_complete_by_type()
to include/net/gro.h where they belong.

Also use _NET_GRO_H instead of _NET_IPV6_GRO_H to protect
include/net/gro.h from multiple inclusions.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240308102230.296224-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>