virxml: include <libxml/xmlsave.h> for xmlIndentTreeOutput declaration

After libxml2's commit of v2.12.0~101 we no longer get
xmlIndentTreeOutput declaration by us including just
libxml/xpathInternals.h and libxml2's header files leakage.

Resolves: https://bugs.gentoo.org/917516
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/util/virxml.c

@@ -24,6 +24,7 @@
 #include <math.h>               /* for isnan() */
 #include <sys/stat.h>
 
+#include <libxml/xmlsave.h>
 #include <libxml/xpathInternals.h>
 
 #include "virerror.h"