Added pfx files to gitignore (#16472)
The current development documentation on creating self-signed builds of NVDA instructs users to save their PFX file to their NVDA repository root. As this directory is under version control, Git will then recognise the PFX file as a candidate for versioning. However, signing certificates should not be committed to VC.
Description of user facing changes
Git will now ignore PFX files in this repository. There are no end-user facing changes.
Description of development approach
Added *.pfx to .gitignore.
