Fix build for upgrade-unsupported platforms

cmd/syncthing/upgrade_common.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"strconv"
 	"strings"
 )
@@ -16,6 +17,12 @@ type githubAsset struct {
 	Name string `json:"name"`
 }
 
+var (
+	errVersionUpToDate    = errors.New("current version is up to date")
+	errVersionUnknown     = errors.New("couldn't fetch release information")
+	errUpgradeUnsupported = errors.New("upgrade unsupported")
+)
+
 // Returns 1 if a>b, -1 if a<b and 0 if they are equal
 func compareVersions(a, b string) int {
 	arel, apre := versionParts(a)

cmd/syncthing/upgrade_supported.go

@@ -10,7 +10,6 @@ import (
 	"archive/tar"
 	"compress/gzip"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -26,16 +25,7 @@ import (
 
 var GoArchExtra string // "", "v5", "v6", "v7"
 
-var (
-	errVersionUpToDate = errors.New("current version is up to date")
-	errVersionUnknown  = errors.New("couldn't fetch release information")
-)
-
 func upgrade() error {
-	if runtime.GOOS == "windows" {
-		return errors.New("Upgrade currently unsupported on Windows")
-	}
-
 	path, err := osext.Executable()
 	if err != nil {
 		return err

cmd/syncthing/upgrade_unsupp.go

@@ -2,10 +2,6 @@
 
 package main
 
-import "errors"
-
-var errUpgradeUnsupported = errors.New("Automatic upgrade not supported")
-
 func upgrade() error {
 	return errUpgradeUnsupported
 }