include/threads: Fix pthread_mutexattr_t initialization on mtx_init

pthread_attr_init was being erroneously called for initializing an
instance of pthread_mutexattr_t.

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>

include/threads.h

@@ -183,7 +183,7 @@ static inline int mtx_init(FAR mtx_t *mutex, int type)
 
   if (type & mtx_recursive)
     {
-      pthread_attr_init(&attr);
+      pthread_mutexattr_init(&attr);
       pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
       pattr = &attr;
     }