ld/testsuite: add missing definition to PDB test

The file pdb-syms1a.s was missing a definition for T_VOID, which was
causing some types not to be deduplicated. It also meant that the test
couldn't be run against LLVM's lld, which throws an error for this.

This particular test only tests the symbols stream, not the types
stream, which is why the deduplication doesn't result in a change in the
file size.