Addition of a XML schema definition for library configurations

A document type definition can help to work with corresponding files
because it describes the involved data structures to some degree.
Such a format was standardised as a basis for XML documents. Some technical
challenges and limitations were noticed so that another standard like
W3C XML Schema evolved.
https://en.wikipedia.org/wiki/XML_Schema_%28W3C%29

Such a XML schema definition was added so that further improvements will
become easier for the safe data exchange around library configurations for
static source code analysis.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>