Fix C99 compatibility glitch in ogginfo/codec_skeleton.c

Include "utf8.h" for a prototype of the utf8_decode function. This avoids an implicit function declaration and build issues with future compilers.

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC