selftests/kexec: Ignore selftest binary

Add a .gitignore for the test case build object.

Signed-off-by: Dylan Yudaken <dyudaken@gmail.com>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>