processes plugin: fix 3 compiler warnings on OS X

processes plugin: fix 3 compiler warnings on OS X

processes.c:1569:26: warning: comparison of integers of different signs: 'int' and 'mach_msg_type_number_t' (aka 'unsigned int') [-Wsign-compare]
        for (int pset = 0; pset < pset_list_len; pset++)
                           ~~~~ ^ ~~~~~~~~~~~~~
processes.c:1590:27: warning: comparison of integers of different signs: 'int' and 'mach_msg_type_number_t' (aka 'unsigned int') [-Wsign-compare]
                for (int task = 0; task < task_list_len; task++)
                                   ~~~~ ^ ~~~~~~~~~~~~~
processes.c:1688:32: warning: comparison of integers of different signs: 'int' and 'mach_msg_type_number_t' (aka 'unsigned int') [-Wsign-compare]
                        for (int thread = 0; thread < thread_list_len; thread++)
                                             ~~~~~~ ^ ~~~~~~~~~~~~~~~