Kernel: Move Lockable into its own header

Author: https://github.com/boricj
Commit: aea98a85d1
Pull-request: https://github.com/SerenityOS/serenity/pull/8851
Reviewed-by: https://github.com/awesomekling ✅
Reviewed-by: https://github.com/bgianfo

Kernel/FileSystem/FIFO.cpp

@@ -9,6 +9,7 @@
 #include <AK/StdLibExtras.h>
 #include <Kernel/FileSystem/FIFO.h>
 #include <Kernel/FileSystem/FileDescription.h>
+#include <Kernel/Locking/Lockable.h>
 #include <Kernel/Locking/Mutex.h>
 #include <Kernel/Process.h>
 #include <Kernel/Thread.h>