configure.ac: Bump min protobuf version to 2.6.0 for oneof parsing support
configure.ac: Bump min protobuf version to 2.6.0 for oneof parsing support

tutu12777/protobuf-c,tutu12777/protobuf-c,koolhazz/protobuf-c,hongbinz/protobuf-c,y12uc231/protobuf-c,koolhazz/protobuf-c,lanniaoershi/protobuf-c,Sannis/protobuf-c,lanniaoershi/protobuf-c,virqin/protobuf-c,pepsi7959/protobuf-c,y12uc231/protobuf-c,virqin/protobuf-c,Sannis/protobuf-c,virqin/protobuf-c,hongbinz/protobuf-c,koolhazz/protobuf-c,hongbinz/protobuf-c,pepsi7959/protobuf-c,tutu12777/protobuf-c,lanniaoershi/protobuf-c,Sannis/protobuf-c,pepsi7959/protobuf-c,y12uc231/protobuf-c