Skip unicode check for XeLaTeX as well

[PHPirates]

Fixes #1166

Example:

 \documentclass{article}
\usepackage{fontspec}

% Download from https://www.fontsquirrel.com/fonts/computer-modern and put ttf files in /usr/share/fonts/TTF
\setmainfont{CMU Serif}

\begin{document}
    This is a paragraph with a PI (π) in it.
\end{document}