support/testing: test_zfs: add missing libelf dependency

While building the kernel tools, libelf header is missing:

output/TestZfsGlibc/build/linux-5.15.35/tools/objtool/include/objtool/elf.h:10:10: fatal error: gelf.h: No such file or directory
   10 | #include <gelf.h>

Select BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF to build host-libelf.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/2429014008

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

support/testing/tests/package/test_zfs.py

@@ -15,6 +15,7 @@ class TestZfsBase(infra.basetest.BRTest):
         BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.15.35"
         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_ZFS=y
         BR2_PACKAGE_PYTHON3=y
         BR2_PACKAGE_PYTHON_CFFI=y