Reproducible builds: drop version from XMLGUI_COMPILING_OS

This is a fix for the Debian bug: https://bugs.debian.org/835053. The
reproducible build team found kxmlgui not reproducible by the use of
the CMAKE_SYSTEM variable. Changing it to CMAKE_SYSTEM_NAME still gives
us information about the SYSTEM, disregarding the kernel version.

REVIEW: 130069