[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      GuiView.o
GuiView.cpp: In member function 'void lyx::frontend::GuiView::updateColorsModel() const':
GuiView.cpp:1305:7: error: 'qsizetype' was not declared in this scope
  for (qsizetype i = 0; i < cis.size(); ++i) {
       ^~~~~~~~~
GuiView.cpp:1305:7: note: suggested alternative: 'size_type'
  for (qsizetype i = 0; i < cis.size(); ++i) {
       ^~~~~~~~~
       size_type
GuiView.cpp:1305:24: error: 'i' was not declared in this scope
  for (qsizetype i = 0; i < cis.size(); ++i) {
                        ^
GuiView.cpp:1305:24: note: suggested alternative: 'id'
  for (qsizetype i = 0; i < cis.size(); ++i) {
                        ^
                        id
Makefile:955: recipe for target 'GuiView.o' failed
make[6]: *** [GuiView.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