examples : add WHISPER_SDL2 check to deprecation executables (#2911)

examples : add WHISPER_SDL2 check to deprecation executables (#2911)

This commit adds a check for `WHISPER_SDL2` to the deprecation warning
examples. This is to prevent the examples from being built when
WHISPER_SDL2 is not enabled.

The motivation for this is that currently these deprecation executables
are generate and when run they refer the user to examples with other
names, for example `whisper-command` but unless they have built with
`WHISPER_SDL2` those executable will not be present:
```console
$ ls build/bin/
bench  command  main  quantize  stream  whisper-bench  whisper-cli
whisper-server

$ ./build/bin/command

WARNING: The binary 'command' is deprecated.
 Please use 'whisper-command' instead.
 See https://github.com/ggerganov/whisper.cpp/tree/master/examples/deprecation-warning/README.md for more information.
```