Fix upstream missing USE_TLS macro guard

[Alkaid-Benetnash]

The current master as of 9ddfcff fails to build without TLS.
It seems that changes from bec3471 were not guarded by the correct macro.

memtier_benchmark.cpp:320:15: error: no member named 'tls_protocols' in 'benchmark_config'
  320 |     if (!cfg->tls_protocols)                                               
      |          ~~~  ^                                                            
memtier_benchmark.cpp:321:14: error: no member named 'tls_protocols' in 'benchmark_config'
  321 |         cfg->tls_protocols = REDIS_TLS_PROTO_DEFAULT;                      
      |         ~~~  ^                                                             
memtier_benchmark.cpp:321:30: error: use of undeclared identifier 'REDIS_TLS_PROTO_DEFAULT'
  321 |         cfg->tls_protocols = REDIS_TLS_PROTO_DEFAULT;                      
      |                              ^                                             
memtier_benchmark.cpp:330:16: warning: variable 'ignore' set but not used [-Wunused-but-set-variable]
  330 |         size_t ignore = fread(&R, sizeof(R), 1, f);                        
      |                ^ 

[filipecosta90]

Thank you @Alkaid-Benetnash ! Approving.
PS: added a build variant to track/avoid this issue in the future.