Added a placeholed for sample/mobilewebapp/war. Required by source target.

Review at http://gwt-code-reviews.appspot.com/1451806


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10256 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/samples/mobilewebapp/war/.place-holder b/samples/mobilewebapp/war/.place-holder
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/samples/mobilewebapp/war/.place-holder