prevent failure in class_list.cpp when LAPACK is not present

Introduced by PR #2292 which causes class_list.cpp to fail due to incorrectly considering CMMDKernelSelectionCombOpt and CMMDKernelSelectionCombMaxL2 as non-abstract classes in absence of LAPACK. Caught [here](http://buildbot.shogun-toolbox.org/builders/bsd1%20-%20libshogun/builds/2339/steps/compile/logs/stdio).