grpc plugin: Fix compile error.

Weirdly, this only surfaces when building with CFLAGS="-O0 -g".

In file included from ./daemon/common.h:33:0,
                 from grpc.cc:44:
grpc.cc: In member function 'virtual grpc::Status CollectdImpl::PutValues(grpc::ServerContext*, grpc::ServerReader<collectd::PutValuesRequest>*, collectd::PutValuesResponse*)':
./daemon/plugin.h:113:56: sorry, unimplemented: non-trivial designated initializers not supported
 #define VALUE_LIST_INIT { .values = NULL, .meta = NULL }
                                                        ^
grpc.cc:294:22: note: in expansion of macro 'VALUE_LIST_INIT'
    value_list_t vl = VALUE_LIST_INIT;
                      ^