deprecated warning

[patcmorneau]

OpenSidescan/src/OpenSidescan/ui/mainimage/imagetab.cpp:95:55: warning: ‘const QPixmap* QLabel::pixmap() const’ is deprecated: Use the other overload which returns QPixmap by-value [-Wdeprecated-declarations]
   95 |             const QPixmap *pixmap = imageLabel->pixmap();