Create war artifact

web.xml is restored from: Ib8f187039.  Ths commit partially reverts
f54665e0a0ec4025fa01d79911ff5ae4e88fc19f.

Test Plan:

  $ bazel build gitiles
  $ unzip -t bazel-bin/gitiles.war

Change-Id: I9778da62b06b357c718275837a3df95ca0043562

gitiles-war/BUILD

diff --git a/gitiles-war/BUILD b/gitiles-war/BUILD
new file mode 100644
index 0000000..be3000e
--- /dev/null
+++ b/gitiles-war/BUILD
@@ -0,0 +1,5 @@
+filegroup(
+    name = "web_xml",
+    srcs = ["src/main/resources/web.xml"],
+    visibility = ["//visibility:public"],
+)