riscv: tests: Make RISCV_KPROBES_KUNIT tristate

riscv: tests: Make RISCV_KPROBES_KUNIT tristate

This disallows KUNIT=m and RISCV_KPROBES_KUNIT=y, which produces these
relocs_check.sh warnings when RELOCATABLE=y:

    WARNING: 3 bad relocations
    ffffffff81e24118 R_RISCV_64        kunit_unary_assert_format
    ffffffff81e24a60 R_RISCV_64        kunit_binary_assert_format
    ffffffff81e269d0 R_RISCV_JUMP_SLOT  __kunit_do_failed_assertion

This fixes allmodconfig build.

Reported-by: Inochi Amaoto <inochiama@gmail.com>
Fixes: f2fab612824f ("riscv: Add kprobes KUnit test")
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Tested-by: Inochi Amaoto <inochiama@gmail.com>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/r/20251020-riscv-kunit-kconfig-fix-6-18-v1-2-d773b5d5ce48@iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>