fix: Qt 6.11 compatibility for QCompleter include

Fixes the following build failure on Arch:

```
src/widgets/dsearchcombobox.cpp:46:49: error: invalid use of incomplete type ‘class QCompleter’
```

Author: felixonmars

src/widgets/dsearchcombobox.cpp

@@ -16,6 +16,7 @@
 #include <QRegularExpression>
 #include <QRegularExpressionValidator>
 #include <QLayout>
+#include <QCompleter>
 
 DWIDGET_BEGIN_NAMESPACE