LibC: Define offsetof in stddef.h instead of sys/cdefs.h

Just like Dr. POSIX ordered. Move the macro where it belongs, but make
sure it's visible to Userland files compiled with -DKERNEL. Parts of
LibEDID that are compiled into the Kernel use offsetof to parse the
EDID information given to us by hardware.

Userland/Libraries/LibC/stddef.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#define offsetof(type, member) __builtin_offsetof(type, member)
+
 #ifndef KERNEL
 
 #    include <sys/cdefs.h>

Userland/Libraries/LibC/sys/cdefs.h

@@ -22,5 +22,3 @@
 
 #undef __P
 #define __P(a) a
-
-#define offsetof(type, member) __builtin_offsetof(type, member)