Bug#862808: libqglviewer-headers: wrong include in /usr/include/QGLViewer/vec.h, line 29

[Bastian Germann]

Control: tags -1 moreinfo

You should only expect things to work when you are setting them up as
intended. The CFLAGS for your project should include the necessary -I
flag to point the preprocessor to the right place.

Maybe the libQGLViewer-qt5.prl needs to be changed to achieve this.
But for now you can work around by checking QtXml via pkgconf.
Does that work for you?