Fix relative path of bpf modules

Using bpf as test data installs the module into a strange
subdirectory, clear the Rel() value before exporting it as
a SourceFileProducer.

Bug: 130206035
Test: m vts_test_binary_bpf_module
Change-Id: Ifdae90d11b15d65c097562d7d6985a8a1dca2bfd