csim: Fix C23 compatability warning

The plat_get_16_random_bits was missing its unit argument, which
produces the following warning:

```
generated_definitions/c/riscv_model_RV64.c:28041:34: warning: passing arguments to 'plat_get_16_random_bits' without a prototype is deprecated in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]
  zseed = plat_get_16_random_bits(UNIT);
```

This commit adds the appropriate argument to the function in the C simulator