Fix bug in the way pthead_mutex_t was being destroyed in win32.

Win32 critical section objects (CRITICAL_SECTION) are not kernel objects.
Only kernel objects are destroyed by using CloseHandle.  Critical sections
are supposed to be deleted with the DeleteCriticalSection API
(http://msdn.microsoft.com/en-us/library/ms682552(VS.85).aspx).

src/win32/pthread.c

@@ -55,9 +55,8 @@ int pthread_mutex_init(pthread_mutex_t *GIT_RESTRICT mutex,
 
 int pthread_mutex_destroy(pthread_mutex_t *mutex)
 {
-    int ret;
-    ret = CloseHandle(mutex);
-    return -(!ret);
+    DeleteCriticalSection(mutex);
+    return 0;
 }
 
 int pthread_mutex_lock(pthread_mutex_t *mutex)