support/testing: TestSELinuxSystemd{Ext4, Squashfs} add midding libelf kernel dependency

objtool built by the kernel requires libelf

ldd TestSELinuxSystemdExt4/build/linux-6.1.26/tools/objtool/objtool
  linux-vdso.so.1
  libelf.so.1 => TestSELinuxSystemdExt4/host/lib/libelf.so.1

While updating the kernel used in TestSELinuxSystemd [1] we
forgot to select BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF to provide
Buildroot's host-libelf. Using host-libelf avoid linking with
libelf installed on the host or failing to build objtool if
libelf is not installed.

[1] 60b84fb7ce4612b2b14a675e28331111468e1f2b

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4725186370 (TestSELinuxSystemdSquashfs)
https://gitlab.com/buildroot.org/buildroot/-/jobs/4725186368 (TestSELinuxSystemdExt4)

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/init/test_systemd_selinux.py

@@ -15,6 +15,7 @@ class TestSELinuxSystemd(infra.basetest.BRTest):
         BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.1.26"
         BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
         BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/qemu/x86_64/linux.config"
+        BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF=y
         BR2_PACKAGE_LIBSELINUX=y
         BR2_PACKAGE_REFPOLICY=y
         """