add ifndefs to all headers

src/readlink.hpp

@@ -14,6 +14,9 @@
   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
 
+#ifndef __READLINK_HPP__
+#define __READLINK_HPP__
+
 namespace mergerfs
 {
   namespace fuse
@@ -24,3 +27,5 @@ namespace mergerfs
              size_t      size);
   }
 }
+
+#endif