Fix header only cc_library for bindgen

rust_bindgen() in newer versions of rules_rust no longer accepts header
only cc_library() for the `cc_lib` field. The solution now is to add a
placeholder source file to all existing header libraries that are
passed to rust_bindgen().

Bug: 325494729
Change-Id: I6d06c14bb234b847590e2cc4de25d92fb262bad4