Fix compilation error with Qt 5.15

[spkane]

This fixes the build errors that occur when building with Qt 5.15 (and maybe others).

classes/imageeditor/toolfill.cpp:194:43: error: member access into incomplete type 'const QPainterPath'
  194 |     if (!this->mParameters->selectedPath().isEmpty()) {
      |                                           ^
/opt/homebrew/Cellar/qt@5/5.15.16/lib/QtGui.framework/Headers/qmatrix.h:54:7: note: forward declaration of 'QPainterPath'
   54 | class QPainterPath;
      |       ^
1 error generated.
make: *** [release/linux/.obj/toolfill.o] Error 1

[spkane]

This was also mentioned in a comment here:
https://lcd-image-converter.riuson.com/en/docs/others/build-from-sources/

by: @kfl02

[riuson]

Try build develop branch. This was fixed at rev 551a7d3.

[spkane]

Try build develop branch. This was fixed at rev 551a7d3.

Indeed, the develop branch does build fine. It might still be nice to merge this into main so that it is buildable with current QT 5 releases, but it is good to know that the develop branch is also an option.

Is there a plan to merge develop back into master sometime soon?

[riuson]

I haven't decided yet.
Currently I have Qt only the latest offline version 5.12.12, not later.