Fix clock_gettime

after update crystal i got

```
./bin/crystal 1.cr -O3
/tmp/user/1011/ccwyspM7.o: In function `__crystal_main':
<stdin>:(.text+0x245): undefined reference to `clock_gettime'
<stdin>:(.text+0x3e0): undefined reference to `clock_gettime'
<stdin>:(.text+0x466): undefined reference to `clock_gettime'
<stdin>:(.text+0x552): undefined reference to `clock_gettime'
collect2: выполнение ld завершилось с кодом возврата 1
```

after this fix, is ok.