To fix OSX error:

/Users/travis/build/grooverdan/mariadb-server/storage/xtradb/sync/sync0arr.cc:530:5: warning: format specifies type 'unsigned long' but the argument has type 'os_thread_id_t' (aka '_opaque_pthread_t *') [-Wformat]
                                mutex->thread_id,

                                ^~~~~~~~~~~~~~~~
Signed-off-by: Daniel Black <daniel.black@au.ibm.com>