api: change the license of gobgp.proto from Apache to MIT
api: change the license of gobgp.proto from Apache to MIT

To make it easier for other projects to include gobgp.proto file,
let's use more permissive license, MIT.

[skip ci]

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

osrg/gobgp,fujita/gobgp,osrg/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp,fujita/gobgp,Dipsingh/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp,j-kato/gobgp,tamihiro/gobgp,Dipsingh/gobgp,fujita/gobgp,Dipsingh/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,j-kato/gobgp,fujita/gobgp,tamihiro/gobgp,a16/gobgp