package/grpc: fix cross-compilation with gRPC_BUILD_GRPC_CPP_PLUGIN=OFF

package/grpc: fix cross-compilation with gRPC_BUILD_GRPC_CPP_PLUGIN=OFF

In commit fedf3318e3fd3c9ba57389ed2b36472f1a772b9e, an obsolete patch to
support cross-compilation was removed, in favor of the upstream solution.

However, this caused a small change in behavior: for the target grpc, the
tool 'grpc_cpp_plugin' is now also built, while before it was not.

This tool is only really needed on development machines. Since Buildroot
does not support compilers and such on target itself, the tool is not
needed.

There exists an option gRPC_BUILD_GRPC_CPP_PLUGIN which can be set to 'OFF',
but disabling it in a cross-compilation context yields build failures.

Add a patch to fix that. This patch is intended to be upstreamed to grpc.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>