lxqt-panel: fix for `explicit` (a C++11 keyword) being used as variable

It is needed to override "explicit" as this is a C++ keyword. But it
is used as variable name in xkb.h. This is causing a failure in C++
compile time.  Similar bug here:
https://bugs.freedesktop.org/show_bug.cgi?id=74080

Workaround from
https://github.com/lxde/lxqt-panel/commit/ec62109e0fa678875a9b10fc6f1975267432712d.