kernel: Replace usage of boost::intrusive_ptr with std::shared_ptr for kernel objects. (#3154)

* kernel: Replace usage of boost::intrusive_ptr with std::shared_ptr for kernel objects.

- See https://github.com/citra-emu/citra/pull/4710 for details.

src/core/hle/service/ns/pl_u.cpp

@@ -141,7 +141,7 @@ struct PL_U::Impl {
     }
 
     /// Handle to shared memory region designated for a shared font
-    Kernel::SharedPtr<Kernel::SharedMemory> shared_font_mem;
+    std::shared_ptr<Kernel::SharedMemory> shared_font_mem;
 
     /// Backing memory for the shared font data
     std::shared_ptr<Kernel::PhysicalMemory> shared_font;