riscv: selftests: Add hwprobe binaries to .gitignore

The cbo and which-cpu hwprobe selftests leave their artifacts in the
kernel tree and end up being tracked by git. Add the binaries to the
hwprobe selftest .gitignore so this no longer happens.
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Fixes: a29e2a48 ("RISC-V: selftests: Add CBO tests")
Fixes: ef7d6abb ("RISC-V: selftests: Add which-cpus hwprobe test")
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240425-gitignore_hwprobe_artifacts-v1-1-dfc5a20da469@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

tools/testing/selftests/riscv/hwprobe/.gitignore

 hwprobe
+cbo
+which-cpus