Introduce TERMUX_PKG_INCLUDE_IN_DEVPACKAGE as a method of
specifying additional files for -dev packages, and use that to
add bin/xml2-config (which is needed to e.g. install the python
lxml package) to libxml2-dev.
Should fix https://github.com/termux/termux-app/issues/8
Also change subpackage containing xmllint to libxml2-utils to
match debian.
