Fix compile error in net.c with musl libc

fixes #278

ChangeLog

@@ -2,6 +2,10 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS 2.1.2 released 2015-10-xx
 
+Bugfix
+   * Fix compile error in net.c with musl libc. Found and patch provided by
+     zhasha (#278).
+
 Changes
    * Fixed paths for check_config.h in example config files. (Found by bachp)
      (#291)

library/net.c

@@ -292,7 +292,7 @@ int mbedtls_net_accept( mbedtls_net_context *bind_ctx,
     struct sockaddr_storage client_addr;
 
 #if defined(__socklen_t_defined) || defined(_SOCKLEN_T) ||  \
-    defined(_SOCKLEN_T_DECLARED)
+    defined(_SOCKLEN_T_DECLARED) || defined(__DEFINED_socklen_t)
     socklen_t n = (socklen_t) sizeof( client_addr );
     socklen_t type_len = (socklen_t) sizeof( type );
 #else