package/xerces: add enable network option

package/xerces: add enable network option

Update to add the option to compile xerces with network
enabled by default so it can be unselected to compile
without network support.

When network support is enabled the Network Accessor feature
will decode schema urls and if they don't appear as localhost
or local files, it will open a stream (socket) session with
the remote server. In an embedded setting having the option to
disable this allows:
 * cleaner audit logging
 * smaller security attack surface
 * less library dependencies
 * no behind the scenes failed session attempts

Signed-off-by: Jared Bents <jared.bents@rockwellcollins.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>