Fix prebuilt selection logic for cc_prebuilt_library_headers.

Unlike other prebuilt modules it doesn't have a srcs property of any
kind, so android.Prebuilt cannot defer to the source module just
because its srcsSupplier would return nil.

Test: m nothing
Bug: 202192894
Change-Id: Iafcf165569bad6eae37820cf71aa0fcacb720e02