[Om-synth] fix to make current CVS compile

[Jacob Lee]

The current CVS does not compile for me, but I have a fix to make it
work.

Upon compiling NodeFactory.cpp, it complained about not being able to
find a mysterious header <slv2/slv2.h>. The fix is to surround that
#include with #ifdef HAVE_LV2 ... #endif, in the same manner that the
other LV2 code is conditioned out. After that, it compiled fine.

I definitely have enjoyed playing around with Om, by the way :-). I
think it has a lot of possibilities for really interesting sounds. I
haven't enjoyed the segfaults as much, of course, but if I can reproduce
any consistently in the latest CVS, I'll try to file bug reports.