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: edae34a3 ("selftests net: add UDP GRO fraglist + bpf self-tests")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Lina Wang <lina.wang@mediatek.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20220606064517.8175-1-lina.wang@mediatek.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/bpf/Makefile

@@ -2,6 +2,7 @@
 
 CLANG ?= clang
 CCINCLUDE += -I../../bpf
+CCINCLUDE += -I../../../lib
 CCINCLUDE += -I../../../../../usr/include/
 
 TEST_CUSTOM_PROGS = $(OUTPUT)/bpf/nat6to4.o
@@ -10,5 +11,4 @@ all: $(TEST_CUSTOM_PROGS)
 $(OUTPUT)/%.o: %.c
 	$(CLANG) -O2 -target bpf -c $< $(CCINCLUDE) -o $@
 
-clean:
-	rm -f $(TEST_CUSTOM_PROGS)
+EXTRA_CLEAN := $(TEST_CUSTOM_PROGS)