Fix build on FreeBSD.

[aisakovic]

Issue

Building the release branch (v6.0.0) fails on FreeBSD.

This is the error:

/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/src/tools/io-benchmark.cpp:112:65: error: use of undeclared identifier 'write_1gb_stop'
  112 |         osrm::util::Log(logDEBUG) << "writing raw 1GB took " << TIMER_SEC(write_1gb) << "s";
      |                                                                 ^
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/include/util/timing_util.hpp:20:60: note: expanded from macro 'TIMER_SEC'
   20 |      std::chrono::duration_cast<std::chrono::microseconds>(_X##_stop - _X##_start).count())
      |                                                            ^
<scratch space>:309:1: note: expanded from here
  309 | write_1gb_stop
      | ^
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/src/tools/io-benchmark.cpp:112:65: error: use of undeclared identifier 'write_1gb_start'
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/include/util/timing_util.hpp:20:72: note: expanded from macro 'TIMER_SEC'
   20 |      std::chrono::duration_cast<std::chrono::microseconds>(_X##_stop - _X##_start).count())
      |                                                                        ^
<scratch space>:310:1: note: expanded from here
  310 | write_1gb_start
      | ^
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/src/tools/io-benchmark.cpp:114:44: error: use of undeclared identifier 'write_1gb_stop'
  114 |                           << 1024 * 1024 / TIMER_SEC(write_1gb) << "MB/sec";
      |                                            ^
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/include/util/timing_util.hpp:20:60: note: expanded from macro 'TIMER_SEC'
   20 |      std::chrono::duration_cast<std::chrono::microseconds>(_X##_stop - _X##_start).count())
      |                                                            ^
<scratch space>:311:1: note: expanded from here
  311 | write_1gb_stop
      | ^
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/src/tools/io-benchmark.cpp:114:44: error: use of undeclared identifier 'write_1gb_start'
/usr/ports/www/osrm-backend/work/osrm-backend-6.0.0/include/util/timing_util.hpp:20:72: note: expanded from macro 'TIMER_SEC'
   20 |      std::chrono::duration_cast<std::chrono::microseconds>(_X##_stop - _X##_start).count())
      |                                                                        ^
<scratch space>:312:1: note: expanded from here
  312 | write_1gb_start
      | ^
4 errors generated. 

The issue is that calls to the TIMER_START and TIMER_STOP macros are required in order to initialize the values of write_1gb_start and write_1gb_stop respectively. Before this PR, these calls occurred only for __APPLE__ and __linux__ features in io-benchmark.cpp.