synchonize ComponentHelper#checkNamespace()

This fixes a bug with use of <parallel> and xmlns:prefix="antlib:package
<project default="bug" xmlns:ac="antlib:net.sf.antcontrib">
  <target name="t2">
    <parallel>
      <ac:shellscript shell="bash">echo hello</ac:shellscript>
      <ac:shellscript shell="bash">echo hello</ac:shellscript>
    </parallel>
  </target>
</project>
- need a unit test for this


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@275522 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/ComponentHelper.java

@@ -806,7 +806,7 @@ public class ComponentHelper  {
      * called for each component name, check if the
      * associated URI has been examined for antlibs.
      */
-    private void checkNamespace(String componentName) {
+    private synchronized void checkNamespace(String componentName) {
         if (componentName.indexOf(':') == -1) {
             return; // not a namespaced name
         }