Add extern "C" to the equeue_stub declaration to avoid an error when a C
file implements the equeue_stub symbol (as we do in equeue_stub.c).

    Undefined symbols for architecture x86_64:
      "_equeue_stub", referenced from:
          Test_LoRaWANTimer_start_Test::TestBody() in Test_LoRaWANTimer.cpp.o
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v to see invocation)