kwin: Fix build failure regarding QElapsedTimer.

It seems not all Qt installs will automatically #include QElapsedTimer
from QtCore/QTimer, this caused a build failure on my system and on a
RHEL 6.2 VM I've been testing on.

Checking the Qt docs, QBasicTimer also has a separate include so
although this didn't cause a build failure, I've thrown in its
separate #include as well. I have not checked for other #include errors,
and a very quick search on b.k.o for bugs mentioning "build" did not
return any bugs to close.

I'm pretty sure this build failure applies only to master but I haven't
checked thoroughly.