package/collectd: snmp_agent needs netsnmp mib loading

package/collectd: snmp_agent needs netsnmp mib loading

snmp_agent plugin was added in version 5.8.0 by
https://github.com/collectd/collectd/commit/8aba9d6e33dea747e4e27e35457f00c287e2c678

It depends on "struct tree" (from net-snmp/library/parse.h) which is
included if BR2_PACKAGE_NETSNMP_ENABLE_MIBS is set

Fix this error by updating configure.ac to check for get_tree in
addition to init_agent

Fixes:
 - http://autobuild.buildroot.org/results/3e24a7671e65974815072f92efaa490d2950b3d9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>