lwtunnel: Fix the sparse warnings in fib_encap_match

When CONFIG_LWTUNNEL config is not enabled, the lwtstate_free() is not
declared in lwtunnel.h at all. However, even in this case, the function
is still referenced in fib_semantics.c so that there appears the
following sparse warnings:

net/ipv4/fib_semantics.c:553:17: error: undefined identifier 'lwtstate_free'
  CC      net/ipv4/fib_semantics.o
  net/ipv4/fib_semantics.c: In function ‘fib_encap_match’:
  net/ipv4/fib_semantics.c:553:3: error: implicit declaration of function ‘lwtstate_free’ [-Werror=implicit-function-declaration]
  cc1: some warnings being treated as errors
  make[1]: *** [net/ipv4/fib_semantics.o] Error 1
  make: *** [net/ipv4/fib_semantics.o] Error 2

To eliminate the error, we define an empty function for lwtstate_free()
in lwtunnel.h when CONFIG_LWTUNNEL is disabled.

Fixes: df383e62 ("lwtunnel: fix memory leak")
Cc: Jiri Benc <jbenc@redhat.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>