ip_tunnel: Protect tunnel functions with CONFIG_INET guard.

Tunnel constants can be used in generic code but in these cases
the inline functions in ip_tunnels.h cause compilation problems
if CONFIG_INET is not set.

CC: Pravin Shelar <pshelar@nicira.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>