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.

grpc.spec

@@ -507,6 +507,7 @@ Summary:        Development files for gRPC library
 # License:        same as base package
 Requires:       grpc%{?_isa} = %{version}-%{release}
 Requires:       grpc-cpp%{?_isa} = %{version}-%{release}
+Requires:       grpc-plugins%{?_isa} = %{version}-%{release}
 
 # grpc/impl/codegen/port_platform.h includes linux/version.h
 Requires:       kernel-headers%{?_isa}