Add filegroup for validate.proto

In order to include this in rust sources this needs to be included
directly downstream. Ideally we had a simple proto library target we
could use instead, but the rust ecosystem is a bit bifurcated.

bazelbuild/rules_rust#915

Signed-off-by: Keith Smiley <keithbsmiley@gmail.com>

validate/BUILD

@@ -53,6 +53,11 @@ java_proto_library(
     deps = [":validate_proto"],
 )
 
+filegroup(
+    name = "validate_src",
+    srcs = ["validate.proto"],
+)
+
 alias(
     name = "go_default_library",
     actual = ":validate_go",