Fix link error in bf_nodes

I got undefined reference errors on the `NodeMFNetworkBuilder::get_default_fn`
function under some circumstances. This symbol is definitely defined in bf_blenkernel.
The error seemed a bit undeterministic and was probably caused by some incorrect
link order. I don't get the error with this change.

source/blender/nodes/CMakeLists.txt

@@ -298,6 +298,7 @@ set(SRC
 )
 
 set(LIB
+  bf_blenkernel
   bf_functions
   bf_intern_sky
 )