Fix warnings

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/18526569830/job/52798726333#step:10:286

  src/libImaging/Arrow.c:224:53: warning: passing 'const char *const' to parameter of type 'char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
    224 |         schema->children[0], im->arrow_band_format, getModeData(im->mode)->name
        |                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  src/libImaging/Arrow.c:152:67: note: passing argument to parameter 'name' here
    152 | export_named_type(struct ArrowSchema *schema, char *format, char *name) {
        |                                                                   ^
  1 warning generated.

https://github.com/python-pillow/Pillow/actions/runs/18526569830/job/52798726333#step:10:299

  ld: warning: ignoring duplicate libraries: '-lpil_imaging_mode'

[hugovk]

(I've cancelled some of the CI jobs so the release wheels build can start: https://github.com/python-pillow/Pillow/actions/runs/18527702261. I'll restart these after it's begun.)