Add all the sources to //:srcs filegroup and add a check to detect
missing file to it.

We need to activate this check on presubmits

--
Change-Id: Ia95e92d3816ce92bb69bc0e2cf56e9c60b68d970
Reviewed-on: https://bazel-review.googlesource.com/#/c/3949/
MOS_MIGRATED_REVID=126404792
diff --git a/examples/cpp/BUILD b/examples/cpp/BUILD
index 7c0543e..3048a0d 100644
--- a/examples/cpp/BUILD
+++ b/examples/cpp/BUILD
@@ -26,8 +26,5 @@
 
 filegroup(
     name = "srcs",
-    srcs = ["BUILD"] + glob([
-        "**/*.cc",
-        "**/*.h",
-    ]),
+    srcs = glob(["**"]),
 )