net: secure_seq: Fix warning when CONFIG_IPV6 and CONFIG_INET are not selected

net_secret() is only used when CONFIG_IPV6 or CONFIG_INET are selected.

Building a defconfig with both of these symbols unselected (Using the ARM
at91sam9rl_defconfig, for example) leads to the following build warning:

$ make at91sam9rl_defconfig
#
# configuration written to .config
#

$ make net/core/secure_seq.o
scripts/kconfig/conf --silentoldconfig Kconfig
  CHK     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
make[1]: `include/generated/mach-types.h' is up to date.
  CALL    scripts/checksyscalls.sh
  CC      net/core/secure_seq.o
net/core/secure_seq.c:17:13: warning: 'net_secret_init' defined but not used [-Wunused-function]

Fix this warning by protecting the definition of net_secret() with these
symbols.
Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/secure_seq.c

@@ -10,6 +10,7 @@
 
 #include <net/secure_seq.h>
 
+#if IS_ENABLED(CONFIG_IPV6) || IS_ENABLED(CONFIG_INET)
 #define NET_SECRET_SIZE (MD5_MESSAGE_BYTES / 4)
 
 static u32 net_secret[NET_SECRET_SIZE] ____cacheline_aligned;
@@ -29,6 +30,7 @@ static void net_secret_init(void)
 		cmpxchg(&net_secret[--i], 0, tmp);
 	}
 }
+#endif
 
 #ifdef CONFIG_INET
 static u32 seq_scale(u32 seq)