Kernel: PID/TID typing

This compiles, and contains exactly the same bugs as before.
The regex 'FIXME: PID/' should reveal all markers that I left behind, including:
- Incomplete conversion
- Issues or things that look fishy
- Actual bugs that will go wrong during runtime

Kernel/PerformanceEventBuffer.h

@@ -68,7 +68,7 @@ public:
         return const_cast<PerformanceEventBuffer&>(*this).at(index);
     }
 
-    KBuffer to_json(pid_t, const String& executable_path) const;
+    KBuffer to_json(ProcessID, const String& executable_path) const;
 
 private:
     PerformanceEvent& at(size_t index);