[TSan][libdispatch] Fix failing test

The ThreadSanitizer-x86_64-iossim configuration (different SDK) seems to
require an additional #include.

rdar://49856637

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@358300 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed