grpc chat example doesn't install all libraries

After the rewrite of the qtgrpc chat example (https://codereview.qt-project.org/c/qt/qtgrpc/+/578647), it no longer builds correctly on Yocto

ERROR: qtgrpc-6.9.0-r0 do_package_qa: QA Issue: /usr/share/examples/grpc/chat/qtgrpc_chat_client contained in package qtgrpc-examples requires libqtgrpc_chat_client_proto.so, but no providers found in RDEPENDS:qtgrpc-examples? [file-rdeps] 

This is happens since the example doesn't install the qtgrpc_chat_client_proto library and Yocto QA check notices the missing dependency.

Can be easily reproduced on desktop builds with QtCreator by enabling "Staging for installation" in the Build steps.