selftests: landlock: Add the uapi headers include variable

Out of tree build of this test fails if relative path of the output
directory is specified. Add the KHDR_INCLUDES to correctly reach the
headers.
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Mickaël Salaün <mic@linux.microsoft.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/landlock/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -Wall -O2
+CFLAGS += -Wall -O2 $(KHDR_INCLUDES)
 
 src_test := $(wildcard *_test.c)