Updated .gitignore to reject all PDB files.
Updated .gitignore to reject all PDB files.

nanocad-lab/X-Mem,nanocad-lab/X-Mem,Microsoft/X-Mem,mgottscho/X-Mem,mgottscho/X-Mem,nanocad-lab/X-Mem,Microsoft/X-Mem,Microsoft/X-Mem,mgottscho/X-Mem,mgottscho/X-Mem,Microsoft/X-Mem,nanocad-lab/X-Mem