Fix typos in macro names
diff --git a/include/polarssl/threading.h b/include/polarssl/threading.h
index 293da38..a60dc3d 100644
--- a/include/polarssl/threading.h
+++ b/include/polarssl/threading.h
@@ -64,7 +64,7 @@
                        int (*mutex_free)( threading_mutex_t * ),
                        int (*mutex_lock)( threading_mutex_t * ),
                        int (*mutex_unlock)( threading_mutex_t * ) );
-#endif /* POLARSSL_THREADING_ALT_C */
+#endif /* POLARSSL_THREADING_ALT */
 
 /*
  * The function pointers for mutex_init, mutex_free, mutex_ and mutex_unlock