Fix VoiceOver interaction with multiline text components

`convertLineOffset` requires exactly one of `line` and `offset`
parameters to be -1.

[ChangeLog][macOS][Accessibility] VoiceOver now reads all lines in
multiline text components when navigating by lines.

Change-Id: I2872c4f5255a33dd2b493b46b22e672eb5779ee8
Fixes: QTBUG-71563
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoaaccessibilityelement.mm

@@ -351,6 +351,7 @@ static void convertLineOffset(QAccessibleTextInterface *text, int *line, int *of
         if (QAccessibleTextInterface *text = iface->textInterface()) {
             int line = 0; // true for all single line edits
             if (iface->state().multiLine) {
+                line = -1;
                 int position = text->cursorPosition();
                 convertLineOffset(text, &line, &position);
             }