small signed-ness patch from Steve Little Daniel

* xmlschemas.c: small signed-ness patch from Steve Little
Daniel
diff --git a/xmlschemas.c b/xmlschemas.c
index 192cfe8..99bf02b 100644
--- a/xmlschemas.c
+++ b/xmlschemas.c
@@ -1492,7 +1492,7 @@
     if (val == NULL)
         return (NULL);
 
-    if (!strchr(val, ':')) {
+    if (!strchr((char *) val, ':')) {
 	ns = xmlSearchNs(node->doc, node, 0);
 	if (ns) {
 	    *namespace = xmlDictLookup(ctxt->dict, ns->href, -1);