[libc++] Fix forgotten fclose() in unit test

Thanks to Andrey Maksimov for the patch.
Differential Revision: https://reviews.llvm.org/D58732

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355162 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed