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>

tools/bpf/Makefile

@@ -75,7 +75,7 @@ $(OUTPUT)bpf_dbg: $(OUTPUT)bpf_dbg.o
 $(OUTPUT)bpf_asm: $(OUTPUT)bpf_asm.o $(OUTPUT)bpf_exp.yacc.o $(OUTPUT)bpf_exp.lex.o
 	$(QUIET_LINK)$(CC) $(CFLAGS) -o $@ $^
 
-$(OUTPUT)bpf_exp.lex.o: $(OUTPUT)bpf_exp.yacc.c
+$(OUTPUT)bpf_exp.lex.c: $(OUTPUT)bpf_exp.yacc.c
 
 clean: bpftool_clean
 	$(call QUIET_CLEAN, bpf-progs)