selftests: vm: 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.
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/vm/Makefile

@@ -23,7 +23,7 @@ MACHINE ?= $(shell echo $(uname_M) | sed -e 's/aarch64.*/arm64/' -e 's/ppc64.*/p
 # LDLIBS.
 MAKEFLAGS += --no-builtin-rules
 
-CFLAGS = -Wall -I ../../../../usr/include $(EXTRA_CFLAGS)
+CFLAGS = -Wall -I ../../../../usr/include $(EXTRA_CFLAGS) $(KHDR_INCLUDES)
 LDLIBS = -lrt -lpthread
 TEST_GEN_FILES = compaction_test
 TEST_GEN_FILES += gup_test