pthread/mutex: fix a compilation error

pthread/mutex: fix a compilation error

This patch fixes a compilation error below:

    CC:  pthread/pthread_mutexlock.c
    pthread/pthread_mutexlock.c:254:2: error: expected identifier or '(' before 'return'
      return ret;
      ^
    pthread/pthread_mutexlock.c:255:1: error: expected identifier or '(' before '}' token
     }
     ^
    /home/ebsong/tizenrt/os/include/pthread.h: In function 'pthread_mutex_lock':
    pthread/pthread_mutexlock.c:251:2: error: control reaches end of non-void function [-Werror=return-type]
      }
      ^
    cc1: all warnings being treated as errors

Change-Id: I183a97f0cad5a865ec474a73fc525a68766c827b
Signed-off-by: EunBong Song <eunb.song@samsung.com>