devel/okteta: update to 0.26.8

Changes since 0.26.7:
* Fixed: Float 32-bit editor in decoding table tool no longer rejects
  negative values & decimal places
* Fixed: PNG structure definition example now ensures use of big
  endianness
* Improved: translations
* Improved: have more text vertically centered in lists in tool views
* Changed: no more use of fixed font type for any list headers in tool
  views