Fix virJSONValueToString prototype for Mingw32

Fix build on platforms lacking YAJL library by adding missing
'bool pretty' parameter to virJSONValueToString.

src/util/json.c

@@ -1112,7 +1112,8 @@ virJSONValuePtr virJSONValueFromString(const char *jsonstring ATTRIBUTE_UNUSED)
                    _("No JSON parser implementation is available"));
     return NULL;
 }
-char *virJSONValueToString(virJSONValuePtr object ATTRIBUTE_UNUSED)
+char *virJSONValueToString(virJSONValuePtr object ATTRIBUTE_UNUSED,
+                           bool pretty ATTRIBUTE_UNUSED)
 {
     virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                    _("No JSON parser implementation is available"));