kernel-selftest: add missing sources if mm is enabled

Some headers come from 'mm' (in the top source directory, i.e. the root of
Linux's sources), as in the exampels below:

Makefile:LOCAL_HDRS += $(selfdir)/mm/local_config.h $(top_srcdir)/mm/gup_test.h
cow.c:#include "../../../../mm/gup_test.h"
gup_longterm.c:#include "../../../../mm/gup_test.h"
gup_test.c:#include <mm/gup_test.h>
hmm-tests.c:#include <mm/gup_test.h>
uffd-unit-tests.c:#include "../../../../mm/gup_test.h"

Add this directory to KERNEL_SELFTEST_SRC if 'mm' PACKAGECONFIG is enabled.

Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-oe/recipes-kernel/kernel-selftest/kernel-selftest.bb

@@ -80,6 +80,7 @@ KERNEL_SELFTEST_SRC ?= "Makefile \
                         tools \
                         scripts \
                         arch \
+                        ${@bb.utils.filter('PACKAGECONFIG', 'mm', d)} \
                         LICENSES \
 "
 do_compile() {