selftests net: fix bpf build error

bpf_helpers.h has been moved to tools/lib/bpf since 5.10, so add more
including path.

Fixes: edae34a3ed92 ("selftests net: add UDP GRO fraglist + bpf self-tests")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Lina Wang <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Acked-by: Paolo Abeni <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/bpf/Makefile

···
 
 CLANG ?= clang
 CCINCLUDE += -I../../bpf
+CCINCLUDE += -I../../../lib
 CCINCLUDE += -I../../../../../usr/include/
 
 TEST_CUSTOM_PROGS = $(OUTPUT)/bpf/nat6to4.o
···
 $(OUTPUT)/%.o: %.c
 	$(CLANG) -O2 -target bpf -c $< $(CCINCLUDE) -o $@
 
-clean:
-	rm -f $(TEST_CUSTOM_PROGS)
+EXTRA_CLEAN := $(TEST_CUSTOM_PROGS)