ipv6: fix "'ioam6_if_id_max' defined but not used" warn

When compiling without CONFIG_SYSCTL, this warning appears:

  net/ipv6/addrconf.c:99:12: error: 'ioam6_if_id_max' defined but not used [-Werror=unused-variable]
     99 | static u32 ioam6_if_id_max = U16_MAX;
        |            ^~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Simply moving the declaration of this variable under ...

  #ifdef CONFIG_SYSCTL

... with other similar variables fixes the issue.

Fixes: 9ee11f0f ("ipv6: ioam: Data plane support for Pre-allocated Trace")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -96,8 +96,6 @@
 #define IPV6_MAX_STRLEN \
 	sizeof("ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255")
 
-static u32 ioam6_if_id_max = U16_MAX;
-
 static inline u32 cstamp_delta(unsigned long cstamp)
 {
 	return (cstamp - INITIAL_JIFFIES) * 100UL / HZ;
@@ -6550,6 +6548,7 @@ static int addrconf_sysctl_disable_policy(struct ctl_table *ctl, int write,
 
 static int minus_one = -1;
 static const int two_five_five = 255;
+static u32 ioam6_if_id_max = U16_MAX;
 
 static const struct ctl_table addrconf_sysctl[] = {
 	{