Fix compile error in net.c with musl libc

fixes #278
diff --git a/ChangeLog b/ChangeLog
index 68b1255..e3353d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,10 @@
 
 = 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)
diff --git a/library/net.c b/library/net.c
index b5d0688..a77268c 100644
--- a/library/net.c
+++ b/library/net.c
@@ -292,7 +292,7 @@
     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