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/Random.h

@@ -10,6 +10,7 @@
 #include <AK/Assertions.h>
 #include <AK/ByteBuffer.h>
 #include <AK/Types.h>
+#include <Kernel/Locking/Lockable.h>
 #include <Kernel/Locking/Mutex.h>
 #include <Kernel/StdLib.h>
 #include <LibCrypto/Cipher/AES.h>