configure.ac: fix activation of snmp_agent

configure.ac: fix activation of snmp_agent

If netsnmp library has been built without mib loading support (through
--disable-mib-loading), build of snmp_agent fails on:

src/snmp_agent.c: In function 'snmp_agent_get_asn_type':
src/snmp_agent.c:160:47: error: dereferencing pointer to incomplete type 'struct tree'
   return (node != NULL) ? mib_to_asn_type(node->type) : 0;

struct tree is defined in net-snmp/libray/parse.h which is included by
net-snmp/mib_api.h only if NETSNMP_DISABLE_MIB_LOADING is not set.

To fix this error, check for get_tree function in netsnmp library in
addition to init_agent in netsnmpagent library

Fixes:
 - http://autobuild.buildroot.org/results/6d7ac28f154f83208f949c62d28411855f1817f8

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>