package/netsnmp: silence warning when running without IPv6

package/netsnmp: silence warning when running without IPv6

snmpd loudly complains every 3 seconds about a failure reading
/proc/net/if_inet6 if the system does not have IPv6 support:

Jan  1 00:00:12 buildroot daemon.err snmpd[92]: ipaddress_linux: could not open /proc/net/if_inet6: No such file or directory
Jan  1 00:00:15 buildroot daemon.err snmpd[92]: ipaddress_linux: could not open /proc/net/if_inet6: No such file or directory
Jan  1 00:00:18 buildroot daemon.err snmpd[92]: ipaddress_linux: could not open /proc/net/if_inet6: No such file or directory
Jan  1 00:00:21 buildroot daemon.err snmpd[92]: ipaddress_linux: could not open /proc/net/if_inet6: No such file or directory

Add an upstream patch to only print this warning once, rather than on every
poll iteration.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>