Kernel: More PID/TID typing

Author: https://github.com/BenWiederhake
Commit: bee08a4b9f
Pull-request: https://github.com/SerenityOS/serenity/pull/3057
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bgianfo

Libraries/LibCore/ProcessStatisticsReader.h

@@ -33,7 +33,7 @@
 namespace Core {
 
 struct ThreadStatistics {
-    int tid;
+    pid_t tid;
     unsigned times_scheduled;
     unsigned ticks;
     unsigned syscall_count;