fix communicators field access in RunTime class

[UnsavedWings]

This commit fixes print_communicator_statistics() method in RunTime class.

Issue replication steps:

$ cd build
$ cmake .. -DPROTOCOL=3 -DEXTRA="-DPRINT_COMMUNICATOR_STATISTICS"
$ make -j 16

Compiler report:

In file included from /home/user/orq/include/mpc.h:4,
                 from /home/user/orq/include/orq.h:14,
                 from /home/user/orq/bench/micro/micro_merge.cpp:1:
/home/user/orq/include/backend/common/runtime.h: In member function ‘void orq::service::RunTime::print_communicator_statistics()’:
/home/user/orq/include/backend/common/runtime.h:993:33: error: ‘communicators’ was not declared in this scope; did you mean ‘Communicator’?
  993 |             size_t bytes_sent = communicators[i]->getBytesSent();
      |                                 ^~~~~~~~~~~~~
      |                                 Communicator