selftests/bpf: Fix out-of-tree build

mainline inclusion
from mainline-5.12-rc1
commit 5837cede
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5837cedef6f33e01d1c4ad90ee4e966bcc6f9c30

-------------------------------------------------

When building out-of-tree, the .skel.h files are generated into the
$(OUTPUT) directory, rather than $(CURDIR). Add $(OUTPUT) to the include
paths.
Signed-off-by: NJean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-3-jean-philippe@linaro.org
(cherry picked from commit 5837cede)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

tools/testing/selftests/bpf/Makefile

@@ -25,7 +25,7 @@ BPF_GCC		?= $(shell command -v bpf-gcc;)
 SAN_CFLAGS	?=
 CFLAGS += -g -rdynamic -Wall -O2 $(GENFLAGS) $(SAN_CFLAGS)		\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
-	  -I$(TOOLSINCDIR) -I$(APIDIR)					\
+	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT)			\
 	  -Dbpf_prog_load=bpf_prog_test_load				\
 	  -Dbpf_load_program=bpf_test_load_program
 LDLIBS += -lcap -lelf -lz -lrt -lpthread