CMakeLists: Disable `-Wtrigraphs`.

Trigraphs support is generally disabled by default, but some compilers
(e.g. GCC) may still warn when one hypothetical trigraph is encountered.

For example, one of these warnings was introduced as part of #14067:

```
.../Source/Core/DolphinQt/MainWindow.cpp:1433:77: warning: trigraph ??) ignored, use -trigraphs to enable [-Wtrigraphs]
 1433 |       this, tr("Select a File"), dialog_path, tr("All Save States (*.sav *.s??);; All Files (*)"));
      |
.../Source/Core/DolphinQt/MainWindow.cpp:1445:77: warning: trigraph ??) ignored, use -trigraphs to enable [-Wtrigraphs]
 1445 |       this, tr("Select a File"), dialog_path, tr("All Save States (*.sav *.s??);; All Files (*)"));
      |
```

The warning is generally not interesting and has been disabled now.

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