[Mac OS] XKB should not used on Mac OS

[younker]

Hi,
With today's svn version, the XKB.cpp is included in the build process
of konsole on Mac OS X.
I think there should be a test case to check whether this is X11
platform, if not, should not include
XKB.cpp in $kdebase/apps/konsole/src/CMakefile.txt

Regards.
mac.younker at gmail.com