qmltermwidget doesn't build on v21.03

Build fails with a warning. Looks like we need to disable -Werror:

lib/TerminalDisplay.cpp: In member function 'virtual bool Konsole::TerminalDisplay::event(QEvent*)':
lib/TerminalDisplay.cpp:2926:54: warning: this statement may fall through [-Wimplicit-fallthrough=]
 2926 |         _scrollBar->setPalette( QApplication::palette() );
      |                                 ~~~~~~~~~~~~~~~~~~~~~^~
lib/TerminalDisplay.cpp:2927:5: note: here
 2927 |     case QEvent::InputMethodQuery:
      |     ^~~~
>>> ERROR: qmltermwidget: build failed

https://builds.sr.ht/~postmarketos/job/434429#task-pmbootstrap_build

@PureTryOut, can you look into it? Upgrading to a newer version would also be an option, since we want to upgrade to the latest plasma on v21.03 anyways.