Add missing import to WritingStep.qml

[uro666]

The WritingStep.qml file is missing an import for QtQuick.Controls.Material which causes the application to throw a Non-existent attached object exception for the ProgressBar in WritingStep and ultimately rpi-imager fails to run through pkexec or its .desktop file:

$ pkexec /usr/bin/rpi-imager 
Running as root via pkexec
Original user: uro
Original UID: 1001
Original home directory: /home/uro
DISPLAY already set to: :0
XAUTHORITY already set to: /run/user/1001/xauth_ygrJgv
Set HOME to: /home/uro
Set XDG_CACHE_HOME to: /home/uro/.cache
Set XDG_CONFIG_HOME to: /home/uro/.config
Set XDG_DATA_HOME to: /home/uro/.local/share
Set XDG_RUNTIME_DIR to: /run/user/1001
Set DBUS_SESSION_BUS_ADDRESS to: unix:path=/run/user/1001/bus
No D-Bus session bus available
QQmlApplicationEngine failed to load component
qrc:/qt/qml/RpiImager/main.qml:107:9: Type WizardContainer unavailable
qrc:/qt/qml/RpiImager/wizard/WizardContainer.qml:1066:9: Type WritingStep unavailable
qrc:/qt/qml/RpiImager/wizard/WritingStep.qml:327:17: Non-existent attached object
Qt has caught an exception thrown from an event handler. Throwing
exceptions from an event handler is not supported in Qt.
You must not let any exception whatsoever propagate through Qt code.

(process:162315): GLib-CRITICAL **: 17:50:38.315: g_source_unref_internal: assertion 'source != NULL' failed

With the patch applied the rpi-imager application runs successfully being launched from the terminal with pkexec or with its .desktop file ( which runs rpi-imager via pkexec ) and does not throw any exceptions:

 $ pkexec /usr/bin/rpi-imager 
Running as root via pkexec
Original user: uro
Original UID: 1001
Original home directory: /home/uro
DISPLAY already set to: :0
XAUTHORITY already set to: /run/user/1001/xauth_ygrJgv
Set HOME to: /home/uro
Set XDG_CACHE_HOME to: /home/uro/.cache
Set XDG_CONFIG_HOME to: /home/uro/.config
Set XDG_DATA_HOME to: /home/uro/.local/share
Set XDG_RUNTIME_DIR to: /run/user/1001
Set DBUS_SESSION_BUS_ADDRESS to: unix:path=/run/user/1001/bus
No D-Bus session bus available
OSListModel: Pruning OS entry with no valid subitems: "RetroPie"
OSListModel: Pruning OS entry with no valid subitems: "Batocera.linux"
OSListModel: Pruning OS entry with no valid subitems: "IPFire"
OSListModel: Pruning OS entry with no valid subitems: "vetra player"