Add dependency on grpc-plugins from grpc-devel
    
    This fixes errors like:
    
    > CMake Error at /usr/lib64/cmake/grpc/gRPCTargets.cmake:182 (message):
    >   The imported target "gRPC::grpc_cpp_plugin" references the file
    >
    >      "/usr/bin/grpc_cpp_plugin"
    >
    >   but this file does not exist.  Possible reasons include:
    >
    >   * The file was deleted, renamed, or moved to another location.
    >
    >   * An install or uninstall procedure did not complete successfully.
    >
    >   * The installation package was faulty and contained
    >
    >      "/usr/lib64/cmake/grpc/gRPCTargets.cmake"
    >
    >   but not all the files it references.