[process]: move ErrorNoChildren to platform independent.

process/process.go

@@ -3,6 +3,7 @@ package process
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"runtime"
 	"time"
 
@@ -11,7 +12,10 @@ import (
 	"github.com/shirou/gopsutil/mem"
 )
 
-var invoke common.Invoker = common.Invoke{}
+var (
+	invoke          common.Invoker = common.Invoke{}
+	ErrorNoChildren                = errors.New("process does not have children")
+)
 
 type Process struct {
 	Pid            int32 `json:"pid"`

process/process_linux.go

@@ -7,7 +7,6 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io/ioutil"
 	"math"
@@ -23,10 +22,7 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-var (
-	ErrorNoChildren = errors.New("process does not have children")
-	PageSize        = uint64(os.Getpagesize())
-)
+var PageSize = uint64(os.Getpagesize())
 
 const (
 	PrioProcess = 0   // linux/resource.h