[TSan][libdispatch] Enable test that supposedly deadlocks on bot

Re-enable test that was disabled because it deadlocks when running on
the bot, but was never enabled again. Can't reproduce deadlock locally
so trying to investigate by re-enabling test.

llvm-svn: 360388

compiler-rt/test/tsan/libdispatch/io-race.c

@@ -1,8 +1,6 @@
 // RUN: %clang_tsan %s -o %t
 // RUN: %deflake %run %t 2>&1 | FileCheck %s
 
-// REQUIRES: disabled
-
 #include <dispatch/dispatch.h>
 
 #include "../test.h"