selftests/landlock: fix broken include of linux/landlock.h

Revert part of the earlier changes to fix the kselftest build when
using a sub-directory from the top of the tree as this broke the
landlock test build as a side-effect when building with "make -C
tools/testing/selftests/landlock".

Reported-by: Mickaël Salaün <mic@digikod.net>
Fixes: a917dd94 ("selftests/landlock: drop deprecated headers dependency")
Fixes: f2745dc0 ("selftests: stop using KSFT_KHDR_INSTALL")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/landlock/Makefile

@@ -9,10 +9,13 @@ TEST_GEN_PROGS := $(src_test:.c=)
 TEST_GEN_PROGS_EXTENDED := true
 
 OVERRIDE_TARGETS := 1
+top_srcdir := ../../../..
 include ../lib.mk
 
+khdr_dir = $(top_srcdir)/usr/include
+
 $(OUTPUT)/true: true.c
 	$(LINK.c) $< $(LDLIBS) -o $@ -static
 
-$(OUTPUT)/%_test: %_test.c ../kselftest_harness.h common.h
-	$(LINK.c) $< $(LDLIBS) -o $@ -lcap
+$(OUTPUT)/%_test: %_test.c $(khdr_dir)/linux/landlock.h ../kselftest_harness.h common.h
+	$(LINK.c) $< $(LDLIBS) -o $@ -lcap -I$(khdr_dir)