Fix: added <cmath>

[Jamaika1]

fattn-common.cuh: In function 'void flash_attn_mask_to_KV_max(const half2*, int*, int, int, int)':
fattn-common.cuh:599:38: error: there are no arguments to 'isinf' that depend on a template parameter, so a declaration of 'isinf' must be available [-Wtemplate-body]
  599 |             all_inf = all_inf && int(isinf(tmp.x)) && int(isinf(tmp.y));
      |                                      ^~~~~
fattn-common.cuh:599:38: note: (if you use '-fpermissive', G++ will accept your code, but allowing the use of an undeclared name is deprecated)
fattn-common.cuh:599:59: error: there are no arguments to 'isinf' that depend on a template parameter, so a declaration of 'isinf' must be available [-Wtemplate-body]
  599 |             all_inf = all_inf && int(isinf(tmp.x)) && int(isinf(tmp.y));
      |                                                           ^~~~~