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.
```

examples/deprecation-warning/CMakeLists.txt

@@ -1,4 +1,6 @@
 add_executable(main ./deprecation-warning.cpp)
 add_executable(bench ./deprecation-warning.cpp)
-add_executable(stream ./deprecation-warning.cpp)
-add_executable(command ./deprecation-warning.cpp)
+if (WHISPER_SDL2)
+    add_executable(stream ./deprecation-warning.cpp)
+    add_executable(command ./deprecation-warning.cpp)
+endif()