selftests: kvm: add amx_test to .gitignore

amx_test's binary should be present in the .gitignore file for the git
to ignore it.

Fixes: bf70636d ("selftest: kvm: Add amx selftest")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Message-Id: <20220118122053.1941915-1-usama.anjum@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>