Revert "netsnmp: bump to 5.7.1 LTS"

Gustavoz reported that 5.7 series segfault on powerpc.

This reverts commit 60859a1e1ac717aafc3543f92adeccba9ed33b29.

package/netsnmp/netsnmp.mk

@@ -4,7 +4,7 @@
 #
 #############################################################
 
-NETSNMP_VERSION = 5.7.1
+NETSNMP_VERSION = 5.6.1.1
 NETSNMP_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/net-snmp
 NETSNMP_SOURCE = net-snmp-$(NETSNMP_VERSION).tar.gz
 NETSNMP_INSTALL_STAGING = YES