FindXercesC: Correct order of version detection and versioned library checks

Further testing showed an issue with !2210 (merged) which is fixed here. Not sure why I didn't pick up on this when reviewing the first time. The major and minor versions used for detecting the Windows libraries aren't set until after these checks are done, which results in the libraries not being found. Moving the version check above to where it is in FindXalanC results in success.

/cc @metsma