selftests: add .gitignore file for RISC-V hwprobe

The executable file "hwprobe" should be ignored by git, adding it to fix
that.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20230605110724.21391-28-andy.chiu@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>