kern: implement KMemoryManager init

libraries/libmesosphere/source/kern_kernel.cpp

@@ -22,6 +22,7 @@ namespace ams::kern {
     KThread          Kernel::s_main_threads[cpu::NumCores];
     KThread          Kernel::s_idle_threads[cpu::NumCores];
     KResourceLimit   Kernel::s_system_resource_limit;
+    KMemoryManager   Kernel::s_memory_manager;
 
     void Kernel::InitializeCoreLocalRegion(s32 core_id) {
         /* Construct the core local region object in place. */