Add missing func GetCacheDir()

restic now compiles on windows

cache_windows.go

@@ -0,0 +1,45 @@
+package restic
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/restic/restic/debug"
+)
+
+// GetCacheDir returns the cache directory
+func GetCacheDir() (string, error) {
+	AppData := os.Getenv("AppData")
+
+	if AppData == "" {
+		return "", errors.New("unable to locate cache directory")
+	}
+
+	cachedir := ""
+	if AppData != "" {
+		cachedir = filepath.Join(AppData, "restic")
+	}
+
+	fi, err := os.Stat(cachedir)
+	if os.IsNotExist(err) {
+		err = os.MkdirAll(cachedir, 0700)
+		if err != nil {
+			return "", err
+		}
+
+		fi, err = os.Stat(cachedir)
+		debug.Log("getCacheDir", "create cache dir %v", cachedir)
+	}
+
+	if err != nil {
+		return "", err
+	}
+
+	if !fi.IsDir() {
+		return "", fmt.Errorf("cache dir %v is not a directory", cachedir)
+	}
+
+	return cachedir, nil
+}