Factor out hard link code for cross platform build

Move system specific code out of entry so the build continues to work on
Windows. Hard links are not currently supported on Windows, the behavior
is a no-op.

src/duplicacy_utils_windows.go

@@ -110,6 +110,12 @@ func SetOwner(fullPath string, entry *Entry, fileInfo *os.FileInfo) bool {
 	return true
 }
 
+type listEntryLinkKey struct{}
+
+func (entry *Entry) getHardLinkKey(f os.FileInfo) (key listEntryLinkKey, linked bool) {
+	return
+}
+
 func (entry *Entry) ReadAttributes(top string) {
 }