Consider '.syso' as a Go file for vendoring

This fixes an issue where importing a package in order to bundle a .syso
doesn't work, because vndr deletes the .syso file when cleaning.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

clean.go

@@ -51,7 +51,7 @@ func isInterestingDir(path string) bool {
 
 func isGoFile(path string) bool {
 	ext := filepath.Ext(path)
-	return ext == ".go" || ext == ".c" || ext == ".h" || ext == ".s" || ext == ".proto"
+	return ext == ".go" || ext == ".c" || ext == ".h" || ext == ".s" || ext == ".proto" || ext == ".syso"
 }
 
 // licenseFilesRegexp is a regexp of file names that are likely to contain licensing information