main/protobuf-c: fix wrong license

It doesn't look like this project has ever been licensed under GPL
license (see [1]).

[1]: https://github.com/protobuf-c/protobuf-c/commit/2689e0625719bc529018bea1215f027b92b78500#diff-04c6e90faac2675aa89e2176d2eec7d8