sprintf is deprecated on macOS 13

[aeiouaeiouaeiouaeiouaeiouaeiou]

make[2]: Entering directory `/opt/local/var/macports/build/_Users_runner_work_macports-ports_macports-ports_ports_audio_audiowmark/audiowmark/work/audiowmark-0.6.1/src'
    CXX      audiowmark.o
    CXX      utils.o
    CXX      random.o
    CXX      convcode.o
  utils.cc:117:7: warning: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
        sprintf (buffer, "%02x", byte);
        ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk/usr/include/stdio.h:188:1: note: 'sprintf' has been explicitly marked deprecated here
  __deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
  ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk/usr/include/sys/cdefs.h:215:48: note: expanded from macro '__deprecated_msg'
          #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))
                                                        ^
  1 warning generated.

This is an uncritical but strange decision to force snprintf().

[swesterfeld]

Ok, the current code is not really broken, but on the other hand, we have already a "safe" version of sprintf called string_printf, which doesn't require guessing an upper bound, so I'll change the code to just use this.