Define bernoulli_cache_num as ulong

This fixes the following warning in the header file:

/home/certik/repos/hashstack/default/include/bernoulli.h: In function ‘void
_bernoulli_fmpq_ui(fmpz*, fmpz*, mp_limb_t)’:
/home/certik/repos/hashstack/default/include/bernoulli.h:131:11: warning:
comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (n < bernoulli_cache_num)
                ^

Arb compiles with this change and seems to work.