CMakeLists.txt: Use cmake's automoc

This fixes a error with cmakes >= 2.8.12: 'The dependency target
"kmixctrl_automoc" of target "kmixctrl" does not exist...'

I guess 48f6e64d98e4aaa47f2a49faa5f3e821bc21fd6e in automoc.git would
also fix the error, but since kmix already depends on cmake 2.8.11,
which has the included automoc, a one-liner seems easier than
expecting people to backport commits for automoc.

REVIEW: 127479