suppress warning from unreachable over-shift

[Smattr]

When debugging is enabled and the state is of non-trivial size, GCC complains about this code:

  In file included from foo.c:7:
  foo.c: In function ‘set_insert’:
  foo.c:3802:40: warning: left shift count >= width of type
    [-Wshift-count-overflow]
   3802 |           assert(*count <= ((size_t)1) << STATE_SIZE_BITS &&
        |                                        ^~

Reported-by: GCC 15.2.1 -Wshift-count-overflow