Fixed key lengths in benchmark and linked math library

[Stemt]

When trying to build the benchmarks I found that the compiler generated the following error:

/home/ar/sources/hashmap.h/bench/sse42.c:50:50: error: ‘snprintf’ output truncated before the last format character [-Werror=format-truncation=]
   50 |     snprintf(keys + (i * key_len), key_len, "%08x", i);
      |                                                  ^
/home/ar/sources/hashmap.h/bench/sse42.c:50:5: note: ‘snprintf’ output 9 bytes into a destination of size 8
   50 |     snprintf(keys + (i * key_len), key_len, "%08x", i);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

I fixed this by adding space for the null terminator by increasing every key_len by 1.

I also found undefined references to 'sqrt' on my linux machine so I added a check that links the math library if it is available.