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)