Update libexpat to upstream release 2.2.0

Signed-off-by: Paul Wassi <p.wassi@gmx.at>

The .so symlinks (without any suffix) are only used by compiler/linker and
not needed on the target system.
See https://github.com/openwrt/packages/pull/274 for a short discussion.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

- fix licensing file list
- install pkgconfig file
- drop explicite call to Build/Compile
- use PKG_INSTALL

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

Signed-off-by: Steven Barth <steven@midlink.org>