selftests: arm64: Fix incorrect kernel headers search path

Use $(KHDR_INCLUDES) as lookup path for kernel headers. This prevents
building against kernel headers from the build environment in scenarios
where kernel headers are installed into a specific output directory
(O=...).

Signed-off-by: Mathieu Desnoyers <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: [e-mail]
Cc: Ingo Molnar <[e-mail]>
Cc: <[e-mail]> # 5.18+
Acked-by: Shuah Khan <[e-mail]>
Acked-by: Catalin Marinas <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/arm64/fp/Makefile

···
 # A proper top_srcdir is needed by KSFT(lib.mk)
 top_srcdir = $(realpath ../../../../../)
 
-CFLAGS += -I$(top_srcdir)/usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := fp-stress \
 	sve-ptrace sve-probe-vls \

tools/testing/selftests/arm64/tags/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -I../../../../../usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 TEST_GEN_PROGS := tags_test
 TEST_PROGS := run_tags_test.sh