tools: bpf: silence make by not deleting intermediate file

Even in quiet mode, make finishes with

rm tools/bpf/bpf_exp.lex.c

That's because it considers the file to be intermediate. Silence that by
mentioning the lex.c file instead of the lex.o file; the dependency still
stays.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>