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.

pkgs/by-name/gh/ghostty/package.nix

@@ -11,9 +11,10 @@
   glslang,
   gtk4-layer-shell,
   harfbuzz,
+  libadwaita,
   libGL,
   libx11,
-  libadwaita,
+  libxml2,
   ncurses,
   nixosTests,
   oniguruma,
@@ -64,6 +65,7 @@ stdenv.mkDerivation (finalAttrs: {
     glib # Required for `glib-compile-schemas`
     wrapGAppsHook4
     blueprint-compiler
+    libxml2 # `xmllint`
   ];
 
   buildInputs = [