forms: Fix issue with Qt 6.8.3 and Clang 21

Clang 21 enabled built-in support for "__yield" but requires including
"arm_acle.h" to work. Qt's "qyieldcpu" header will emit code using the
function without including the declaration and thus fail compilation

A fix for Qt is pending, but AppleClang 21 is shipped since 2026-03-24
with Xcode 26.4 and thus the project cannot be compiled after a
toolchain update otherwise.

Author: PatTheMav

src/forms/SettingsDialog.cpp

@@ -17,7 +17,7 @@ You should have received a copy of the GNU General Public License along
 with this program. If not, see <https://www.gnu.org/licenses/>
 */
 
-#include <QtWidgets/QMessageBox>
+#include <QMessageBox>
 #include <QDateTime>
 #include <QTime>
 #include <obs-module.h>