char: misc: make miscdevice unit test built-in only

char: misc: make miscdevice unit test built-in only

Since it uses __init symbols, it cannot be a module. Builds with
CONFIG_TEST_MISC_MINOR=m will fail with:

ERROR: modpost: "init_mknod" [drivers/misc/misc_minor_kunit.ko] undefined!
ERROR: modpost: "init_unlink" [drivers/misc/misc_minor_kunit.ko] undefined!

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250429155404.2b6fe5b1@canb.auug.org.au/
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504160338.BjUL3Owb-lkp@intel.com/
Fixes: 45f0de4f8dc3 ("char: misc: add test cases")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Link: https://lore.kernel.org/r/20250430-misc-test-fixup-v1-1-6f39ed6c733d@igalia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>