kselftest/arm64: Fix ABI header directory location

Currently the arm64 kselftests attempt to locate the ABI headers using
custom logic which doesn't work correctly in the case of out of tree builds
if KBUILD_OUTPUT is not specified. Since lib.mk defines KHDR_INCLUDES with
the appropriate flags we can simply remove the custom logic and use that
instead.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220503231655.211346-1-broonie@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/Makefile

@@ -17,16 +17,7 @@ top_srcdir = $(realpath ../../../../)
 # Additional include paths needed by kselftest.h and local headers
 CFLAGS += -I$(top_srcdir)/tools/testing/selftests/
 
-# Guessing where the Kernel headers could have been installed
-# depending on ENV config
-ifeq ($(KBUILD_OUTPUT),)
-khdr_dir = $(top_srcdir)/usr/include
-else
-# the KSFT preferred location when KBUILD_OUTPUT is set
-khdr_dir = $(KBUILD_OUTPUT)/kselftest/usr/include
-endif
-
-CFLAGS += -I$(khdr_dir)
+CFLAGS += $(KHDR_INCLUDES)
 
 export CFLAGS
 export top_srcdir