selftests/bpf: remove generated verifier/tests.h on 'make clean'

'make clean' is supposed to remove generated files.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>