relaxng.c: cast to allow compilation with sun studio 11

Sun Studio 11 was failing to compile relaxng.c due to a type mismatch
in a ternary operator used to provide an argument to xmlRngPErr.  It
seems that sos11 cc is more pedantic about this part of the C99
standard.  GCC issues a warning but doesn't fail by default.

After casting the string literal "nothing" to (const xmlChar *) sos11
cc is happy.  GCC no longer issues a warning either.

Another warning was cleaned up with an identical cast.
diff --git a/relaxng.c b/relaxng.c
index 075fc4a..6dbc499 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -5369,7 +5369,8 @@
     } else {
         xmlRngPErr(ctxt, node, XML_RNGP_CHOICE_CONTENT,
                    "expecting name, anyName, nsName or choice : got %s\n",
-                   (node == NULL ? "nothing" : node->name), NULL);
+                   (node == NULL ? (const xmlChar *) "nothing" : node->name),
+		   NULL);
         return (NULL);
     }
     if (ret != def) {
@@ -9459,7 +9460,8 @@
             ctxt->states = NULL;
             if (found == 0) {
                 if (cur == NULL) {
-                    VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA, "noname");
+		    VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA,
+			       (const xmlChar *) "noname");
                 } else {
                     VALID_ERR2(XML_RELAXNG_ERR_INTEREXTRA, cur->name);
                 }