package/libxml2: add upstream security fix for CVE-2019-20388

Fixes CVE-2019-20388: xmlSchemaPreRun in xmlschemas.c in libxml2 2.9.10
allows an xmlSchemaValidateStream memory leak.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>