ipmi: ssif_bmc: Fix KUnit test link failure when KUNIT=m

Building with CONFIG_KUNIT=m and CONFIG_SSIF_IPMI_BMC_KUNIT_TEST=y
results in link errors such as:

undefined reference to `kunit_binary_assert_format'
undefined reference to `__kunit_do_failed_assertion'

This happens because the test code is built-in while the KUnit core
is built as a module, so the required KUnit symbols are not available
at link time.

Fix this by requiring KUNIT to be built-in when enabling
SSIF_IPMI_BMC_KUNIT_TEST.

Signed-off-by: Jian Zhang <zhangjian.3032@bytedance.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604071448.zUBjPYPu-lkp@intel.com/
Message-ID: <20260407094647.356661-1-zhangjian.3032@bytedance.com>
Signed-off-by: Corey Minyard <corey@minyard.net>