intel_rdt: fix format specifier

intel_rdt: fix format specifier

Fixes build on F-26:

src/intel_rdt.c: In function 'rdt_dump_cgroups':
src/intel_rdt.c:34:20: error: format '%d' expects argument of type 'int', but argument 3 has type 'size_t {aka long unsigned int}' [-Werror=format=]
 #define RDT_PLUGIN "intel_rdt"
                    ^
./src/daemon/plugin.h:396:42: note: in definition of macro 'DEBUG'
 #define DEBUG(...) plugin_log(LOG_DEBUG, __VA_ARGS__)
                                          ^~~~~~~~~~~
src/intel_rdt.c:79:11: note: in expansion of macro 'RDT_PLUGIN'
     DEBUG(RDT_PLUGIN ":  group[%d]:", i);
           ^~~~~~~~~~
src/intel_rdt.c:79:33: note: format string is defined here
     DEBUG(RDT_PLUGIN ":  group[%d]:", i);
                                ~^
                                %ld