[automated testing] debian testing: compile failure with qt6

[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
Making all in .
Making all in .
  CHK      lyx_commit_hash.h
  AR       liblyxcore.a
  AR       liblyxmathed.a
  CXX      insets/InsetFloat.o
insets/InsetFloat.cpp: In member function 'virtual lyx::docstring lyx::InsetFloat::xhtml(lyx::XMLStream&, const lyx::OutputParams&) const':
insets/InsetFloat.cpp:376:1: error: control reaches end of non-void function [-Werror=return-type]
  376 | }
      | ^
cc1plus: all warnings being treated as errors
make[4]: *** [Makefile:2421: insets/InsetFloat.o] Error 1
make[3]: *** [Makefile:2625: all-recursive] Error 1
make[2]: *** [Makefile:1701: all] Error 2
make[1]: *** [Makefile:586: all-recursive] Error 1
make: *** [Makefile:481: all] Error 2