Declare 'mutex_list' and 'mutex_list_mutex' extern,
  to avoid link error on OS X and gcc flag "-fno-common"


innobase/include/sync0sync.h:
  Use 'extern' variable declarations, to avoid
  "ld: multiple definitions of symbol" for 'mutex_list'
  and 'mutex_list_mutex' on OS X and gcc flag "-fno-common"
innobase/sync/sync0sync.c:
  Use 'extern' variable declarations, to avoid
  "ld: multiple definitions of symbol" for 'mutex_list'
  and 'mutex_list_mutex' on OS X and gcc flag "-fno-common"