nptl: Avoid using PTHREAD_MUTEX_DEFAULT in macro definition [BZ #25271]

Commit 1c3f9acf ("nptl: Add struct_mutex.h")
replaced a zero constant with the identifier PTHREAD_MUTEX_DEFAULT
in the macro PTHREAD_MUTEX_INITIALIZER.  However, that constant
is not available in ISO C11 mode:

In file included from /usr/include/bits/thread-shared-types.h:74,
                 from /usr/include/bits/pthreadtypes.h:23,
                 from /usr/include/pthread.h:26,
                 from bug25271.c:1:
bug25271.c:3:21: error: ‘PTHREAD_MUTEX_DEFAULT’ undeclared here (not in a function)
    3 | pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~

This commit change the constant to the equivalent
PTHREAD_MUTEX_TIMED_NP, which is in the POSIX extension namespace
and thus always available.