Fix calloc-transposed-args warning

[nobu]

ntt.h: In function ‘ntt_multiply’:
ntt.h:136:41: warning: ‘ruby_xcalloc’ sizes specified with ‘sizeof’ in the earlier argument and not in the later argument [-Wcalloc-transposed-args]
  136 |     uint32_t *mem = ruby_xcalloc(sizeof(uint32_t), ntt_size * 9);
      |                                         ^~~~~~~~
ntt.h:136:41: note: earlier argument should specify number of elements, later size of each element