kernel/object: Tighten object against data races

Despite being covered by a global mutex, we should still ensure that the
class handles its reference counts properly. This avoids potential
shenanigans when it comes to data races.

Given this is the root object that drives quite a bit of the kernel
object hierarchy, ensuring we always have the correct behavior (and no
races) is a good thing.

src/core/hle/kernel/kernel.cpp

@@ -14,7 +14,7 @@
 
 namespace Kernel {
 
-unsigned int Object::next_object_id;
+std::atomic<u32> Object::next_object_id{0};
 
 /// Initialize the kernel
 void Init(u32 system_mode) {