libxml2: fix dependencies and QA Issues

libxml2: fix dependencies and QA Issues

Fix the following QA warnings:

WARNING: libxml2-2.9.3-r0 do_package_qa: QA Issue: libxml2 rdepends on
libiconv, but it isn't a build dependency, missing libiconv in DEPENDS
or PACKAGECONFIG? [build-deps]

WARNING: libxml2-2.9.3-r0 do_package_qa: QA Issue: libxml2-python
rdepends on libiconv, but it isn't a build dependency, missing libiconv
in DEPENDS or PACKAGECONFIG? [build-deps]

Signed-off-by: Maxin B. John <maxin.john@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>