libpng: upgrade 1.6.54 -> 1.6.55

libpng: upgrade 1.6.54 -> 1.6.55

Release notes [1]:
* Fixed CVE-2026-25646 (high severity):
  Heap buffer overflow in `png_set_quantize`.
  (Reported and fixed by Joshua Inscoe.)
* Resolved an oss-fuzz build issue involving nalloc.
  (Contributed by Philippe Antoine.)

[1] https://github.com/pnggroup/libpng/blob/v1.6.55/ANNOUNCE

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>