Treat lib.rs.h as an explicit output

Similar to http://review.skia.org/738779, we were seeing flakiness
on the CI jobs when GN decided it didn't need to call Bazel but also
didn't have the generated header for use.

Bug: b/40045269
Change-Id: I991ce0e652cdca0164f65416923ef6aa08b11f62
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/741436
Reviewed-by: Dominik Röttsches <drott@google.com>
1 file changed