powercap: test counter read permissions

Check that the files inside /sys/class/powercap/intel-rapl:<n>
directories not only exist, but are readable.  On recent Linux kernels,
"energy_uj" is by default readable by root only, which is something that
PAPI fails to detect, resulting in 0 being returned for that counter
without any indication of a problem.

src/components/powercap/linux-powercap.c

@@ -238,7 +238,7 @@ static int _powercap_init_component( int cidx )
       if (strErr > sizeof(event_path)) HANDLE_STRING_ERROR;
       // not a valid pkg event path so continue
 
-      if (access(event_path, F_OK) == -1) { continue; }
+      if (access(event_path, R_OK) == -1) { continue; }
 
       strErr=snprintf(powercap_ntv_events[num_events].name, sizeof(powercap_ntv_events[num_events].name), "%s:ZONE%d", pkg_event_names[e], s);
       powercap_ntv_events[num_events].name[sizeof(powercap_ntv_events[num_events].name)-1]=0;
@@ -286,7 +286,7 @@ static int _powercap_init_component( int cidx )
         if (strErr > sizeof(event_path)) HANDLE_STRING_ERROR;
 
         // not a valid pkg event path so continue
-        if (access(event_path, F_OK) == -1) { continue; }
+        if (access(event_path, R_OK) == -1) { continue; }
 
         strErr=snprintf(powercap_ntv_events[num_events].name, sizeof(powercap_ntv_events[num_events].name), "%s:ZONE%d_SUBZONE%d", component_event_names[e], s, c);
         powercap_ntv_events[num_events].name[sizeof(powercap_ntv_events[num_events].name)-1]=0;