snmp_agent: fix format specifiers

snmp_agent: fix format specifiers

make  all-am
make[1]: Entering directory '/home/ruben/src/collectd'
  CC       src/snmp_agent_la-snmp_agent.lo
In file included from ./src/daemon/common.h:33:0,
                 from src/snmp_agent.c:31:
src/snmp_agent.c: In function ‘snmp_agent_dump_data’:
src/snmp_agent.c:42:21: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
 #define PLUGIN_NAME "snmp_agent"
                     ^
./src/daemon/plugin.h:400:42: note: in definition of macro ‘DEBUG’
 #define DEBUG(...) plugin_log(LOG_DEBUG, __VA_ARGS__)
                                          ^~~~~~~~~~~
src/snmp_agent.c:192:13: note: in expansion of macro ‘PLUGIN_NAME’
       DEBUG(PLUGIN_NAME ":   OID[%d]: %s", i, oid_str);
             ^~~~~~~~~~~
src/snmp_agent.c:192:35: note: format string is defined here
       DEBUG(PLUGIN_NAME ":   OID[%d]: %s", i, oid_str);
                                  ~^
                                  %ld
  CCLD     snmp_agent.la