Compile error on implicit declaration

[Eonfge]

This becomes a problem with the latest version of FreeDesktop SDK 24.08 which has a fairly strict set of rules for GCC 14.

codec_skeleton.c: In function ‘skeleton_process_fisbone_message_header’:
codec_skeleton.c:119:9: error: implicit declaration of function ‘utf8_decode’ [-Wimplicit-function-declaration]
  119 |     if (utf8_decode(header, &decoded) < 0) {
      |         ^~~~~~~~~~~

[Eonfge]

Luckily, there is already a fix: #48