Add x86_64/debug_regs to .gitignore.

Reported-by: Marcelo Bandeira Condotta <mcondotta@redhat.com>
Fixes: 449aa906 ("KVM: selftests: Add KVM_SET_GUEST_DEBUG test")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200608112346.593513-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>