intel_rdt plugin: fix compiler warnings

intel_rdt plugin: fix compiler warnings

make[1]: Entering directory '/home/ruben/src/collectd'
  CC       src/intel_rdt_la-intel_rdt.lo
src/intel_rdt.c: In function ‘rdt_dump_cgroups’:
src/intel_rdt.c:74:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
   74 |     for (int j = 0; j < cgroup->num_cores; j++) {
      |                       ^
src/intel_rdt.c: In function ‘rdt_dump_data’:
src/intel_rdt.c:103:21: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
  103 |   for (int i = 0; i < g_rdt->num_groups; i++) {
      |                     ^
  CCLD     intel_rdt.la