LibC+Kernel: Move device-files related methods to a LibC header file

The Kernel/API directory in general shouldn't include userspace code,
but structure definitions that both are shared between the Kernel and
userspace.

LibC is the most appropriate place for these methods as they're already
included in the sys/sysmacros.h file to create a set of convenient
macros for these methods.

Userland/Libraries/LibC/sys/sysmacros.h

@@ -6,7 +6,7 @@
 
 #pragma once
 
-#include <Kernel/API/Device.h>
+#include <sys/device.h>
 
 #define makedev(major, minor) serenity_dev_makedev((major), (minor))
 #define major(dev) serenity_dev_major(dev)