Build fails with Qt 5.15.9

[antonio-rojas]

Description

With Qt 5.15.9, cmake fails with multiple errors like

  Attempt to add a custom rule to output

    /build/opentoonz/src/build/tnzcore/.lupdate/tnzcore.ts.stamp.rule

  which already has a custom rule.
Call Stack (most recent call first):
  CMakeLists.txt:604 (qt5_create_translation)
  tnzcore/CMakeLists.txt:262 (add_translation)

Steps to Reproduce

1. Update Qt to 5.15.9
2. Attempt to build opentoonz

Expected Behavior

No response

Screenshots, Video & Crash Logs

No response

OpenToonz Version

1.6

OpenToonz Version Information

No response

Operating System

Linux

GPU

Intel

Graphics Tablet

None