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/Net/UDPSocket.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/Singleton.h>
 #include <Kernel/Devices/RandomDevice.h>
+#include <Kernel/Locking/Mutex.h>
 #include <Kernel/Net/NetworkAdapter.h>
 #include <Kernel/Net/Routing.h>
 #include <Kernel/Net/UDP.h>