dependency cycle fix in DynamicLoaderDarwinKernel

DynamicLoaderDarwinKernel calls in to PlatformDarwinKernel, and
with my changes in https://reviews.llvm.org/D133534, PlatformDarwinKernel
calls in to DynamicLoaderDarwinKernel.  This results in a cmake
dependency if accurately included in the link libraries list.

lldbPluginDynamicLoaderDarwinKernel is specfically for kernel
debugging and is uncommonly linked in to anything except a full
lldb.  lldbPluginPlatformMacOSX is any Darwin platform, including
PlatformDarwinKernel, and is referenced a number of time in shell
tests, for instance.

I believe anything linking the darwin kernel DynamicLoader plugin
will already have lldbPluginPlatformMacOSX in its dependency list,
so not explicitly expressing this dependency is safe.