third-party_qemu: modify to .gitignore

N/A

How to make qemu for arm?
1.$ sudo chmod 777 /opt
2.$ ./configure --prefix=/opt/qemu --target-list=arm-softmmu,arm-linux-user --enable-debug
3.$ make
4.$ sudo make install

Signed-off-by: liminghao <liminghao@xiaomi.com>