[automated testing] bionic: compile failure

[testing at tester.lyx.org]

Making all in autotests
Making all in config
Making all in development
Making all in po
Making all in 3rdparty
Making all in nod
Making all in src
  CHK      lyx_commit_hash.h
Making all in support
Making all in frontends
Making all in qt
  CXX      GuiPrefs.o
GuiPrefs.cpp: In member function 'virtual void lyx::frontend::ColorSwatchDelegate::paint(QPainter*, const QStyleOptionViewItem&, const QModelIndex&) const':
GuiPrefs.cpp:4402:16: error: 'QColorConstants' has not been declared
    pixmap.fill(QColorConstants::Transparent);
                ^~~~~~~~~~~~~~~
Makefile:955: recipe for target 'GuiPrefs.o' failed
make[6]: *** [GuiPrefs.o] Error 1
Makefile:795: recipe for target 'all' failed
make[5]: *** [all] Error 2
Makefile:784: recipe for target 'all-recursive' failed
make[4]: *** [all-recursive] Error 1
Makefile:2418: recipe for target 'all-recursive' failed
make[3]: *** [all-recursive] Error 1
Makefile:1500: recipe for target 'all' failed
make[2]: *** [all] Error 2
Makefile:579: recipe for target 'all-recursive' failed
make[1]: *** [all-recursive] Error 1
Makefile:474: recipe for target 'all' failed
make: *** [all] Error 2