collectdctl.c: fix two compiler warnings

collectdctl.c: fix two compiler warnings

collectdctl.c: In function ‘flush’:
collectdctl.c:315:21: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
   for (int i = 0; i < plugins_num; ++i) {
                     ^
collectdctl.c:323:25: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
       for (int j = 0; j < identifiers_num; ++j) {
                         ^