posix/osx: fix type conflict on OSX native

In another header file, `socklen_t` is defined to `__darwin_socklen_t` which is
an `uint32_t` and it conflicts.

Preparation to remove NATIVEINCLUDES.

sys/posix/include/sys/bytes.h

@@ -28,7 +28,12 @@
 extern "C" {
 #endif
 
+#ifndef __MACH__
 typedef size_t socklen_t;           /**< socket address length */
+#else
+/* Defined for OSX with a different type */
+typedef __darwin_socklen_t socklen_t;   /**< socket address length */
+#endif
 
 #ifdef __cplusplus
 }