postgresql : Fix build using pkg-config for libxml-2.0.

Use package-config instead of binconfig for libxml-2.0.

The pgac_option processing is filtering the output from
the xml-config programming. The pkgconfig output is good
enough to use without such filtering.

Signed-off-by: Philip Balister <philip@balister.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>