Without this commit, compilation fails on DragonFly with the following message:

```
/home/mneumann/Dev/benchmark.old/src/sysinfo.cc:446:2: error: #warning "HOST_NAME_MAX not defined. using 64" [-Werror=cpp]
^~~~~~~
```

Also note that the sysctl is actually `hw.tsc_frequency` on DragonFly:

```
$ sysctl hw.tsc_frequency
hw.tsc_frequency: 3498984022
```

Tested on:

```
$ uname -a
DragonFly box.localnet 5.9-DEVELOPMENT DragonFly v5.9.0.742.g4b29dd-DEVELOPMENT #5: Tue Aug 18 00:21:31 CEST 2020
```