qt5: add libxml2-devel to makedepends

Trying to reorder cross-build.
Transitively dependencies via libxslt-devel.

srcpkgs/qt5/template

@@ -11,7 +11,7 @@ makedepends="SDL2-devel Vulkan-Headers alsa-lib-devel double-conversion-devel
  eudev-libudev-devel ffmpeg-devel freetds-devel glib-devel gst-plugins-base1-devel
  gtk+3-devel icu-devel libXv-devel libbluetooth-devel libcap-devel
  libinput-devel libmng-devel libmariadbclient-devel libproxy-devel
- libvpx-devel libwebp-devel libxslt-devel cups-devel minizip-devel
+ libvpx-devel libwebp-devel libxslt-devel libxml2-devel cups-devel minizip-devel
  opus-devel pciutils-devel pcre2-devel postgresql-libs-devel
  protobuf-devel pulseaudio-devel tslib-devel openssl-devel
  unixodbc-devel xcb-util-image-devel xcb-util-keysyms-devel