Allow 4 byte map values for perf_event_array

In 010dc16 the check was added for 8 byte values, which doesn't work for oomkill:

```
Error attaching exporter: error validating maps for config "oomkill": value size for map "oom_kills_total" is not expected 8 bytes (u64), it is 4 bytes
```

exporter/exporter.go

@@ -499,7 +499,7 @@ func validateMaps(module *libbpfgo.Module, cfg config.Config) error {
 	maps := []string{}
 
 	for _, counter := range cfg.Metrics.Counters {
-		if counter.Name != "" {
+		if counter.Name != "" && !counter.PerfEventArray {
 			maps = append(maps, counter.Name)
 		}
 	}