use cross-platform random seed function

The `srandomdev()` function is specific to OSX/BSD. This makes your random seeder cross-platform.

Currently gcc gives the following output on Linux:

```
jay@coltrane:~/github/slowdown$ make
gcc -Wall -O3  slowdown.c -o slowdown
slowdown.c: In function ‘main’:
slowdown.c:82:5: warning: implicit declaration of function ‘srandomdev’ [-Wimplicit-function-declaration]
/tmp/ccepQK9e.o: In function `main':
slowdown.c:(.text.startup+0x172): undefined reference to `srandomdev'
collect2: ld returned 1 exit status
make: *** [slowdown] Error 1
```