selftests/resctrl: Make functions only used in same file static

Fix following sparse warnings:
 tools/testing/selftests/resctrl/resctrl_val.c:47:6: warning: symbol 'membw_initialize_perf_event_attr' was not declared. Should it be static?
 tools/testing/selftests/resctrl/resctrl_val.c:64:6: warning: symbol 'membw_ioctl_perf_event_ioc_reset_enable' was not declared. Should it be
static?
 tools/testing/selftests/resctrl/resctrl_val.c:70:6: warning: symbol 'membw_ioctl_perf_event_ioc_disable' was not declared. Should it be static?
 tools/testing/selftests/resctrl/resctrl_val.c:81:6: warning: symbol 'get_event_and_umask' was not declared. Should it be static?

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>