[libc++] Add missing %link_flags to .sh.cpp test

Without the link flags, the test always fails on Linux. For some reason,
however, it works on Darwin -- which is why it wasn't caught at first.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366579 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed