Make "ant checkstyle" work when no other targets have been run yet.

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11423 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/build.xml b/build.xml
index 37d6f9c..ec0c930 100755
--- a/build.xml
+++ b/build.xml
@@ -128,6 +128,8 @@
     <call-subproject subproject="user" subtarget="checkstyle" />
     <call-subproject subproject="requestfactory" subtarget="checkstyle" />
     <call-subproject subproject="servlet" subtarget="checkstyle" />
+    <!-- servlet needed for tools/benchmark-viewer -->
+    <call-subproject subproject="servlet" subtarget="build" /> 
     <call-subproject subproject="tools" subtarget="checkstyle" />
     <call-subproject subproject="samples" subtarget="checkstyle" />
   </target>