message: Fix a compiler warning: unused GetDevURandom()
  ```
  random.cpp:255:13: error: unused function 'GetDevURandom' [-Werror,-Wunused-function]
  ```
  Clang 9.0.0, FreeBSD 12.1
  Silence by planting a dummy reference to the `GetDevURandom` symbol
  in the places where we don't call the function.
diff: @@ -315,12 +315,16 @@ void GetOSRand(unsigned char *ent32)
  if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
  RandFailure();
  }
  + // Silence a compiler warning about unused function.
  + (void)GetDevURandom;
  #elif defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
  /* getentropy() is available on macOS 10.12 and later.
  */
  if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
  RandFailure();
  }
  + // Silence a compiler warning about unused function.
  + (void)GetDevURandom;
  #elif defined(HAVE_SYSCTL_ARND)
  /* FreeBSD, NetBSD and similar. It is possible for the call to return less
  * bytes than requested, so need to read in a loop.
  @@ -334,6 +338,8 @@ void GetOSRand(unsigned char *ent32)
  }
  have += len;
  } while (have < NUM_OS_RANDOM_BYTES);
  + // Silence a compiler warning about unused function.
  + (void)GetDevURandom;
  #else
  /* Fall back to /dev/urandom if there is no specific method implemented to
  * get system entropy for this OS.
  