ghostty: add libxml2 for xmllint
Somehow I've always managed to miss this, but Ghostty does use xmllint
to minify (`xml-stripblanks`) on the generated GTK .ui files in the
build process. I assume this was never caught as it wasn't mission-
critical and nor did it stop the build process entirely.
(cherry picked from commit 227008f8401e111a0bc784c846ed870bedfaec13)