Remove offsetof() from user-visible <linux/stddef.h>

It's not used by anything user-visible, and it make g++ unhappy.
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

include/linux/Kbuild

@@ -143,7 +143,6 @@ header-y += snmp.h
 header-y += sockios.h
 header-y += som.h
 header-y += sound.h
-header-y += stddef.h
 header-y += synclink.h
 header-y += telephony.h
 header-y += termios.h
@@ -318,6 +317,7 @@ unifdef-y += sonet.h
 unifdef-y += sonypi.h
 unifdef-y += soundcard.h
 unifdef-y += stat.h
+unifdef-y += stddef.h
 unifdef-y += sysctl.h
 unifdef-y += tcp.h
 unifdef-y += time.h

include/linux/stddef.h

@@ -10,11 +10,13 @@
 #define NULL ((void *)0)
 #endif
 
+#ifdef __KERNEL__
 #undef offsetof
 #ifdef __compiler_offsetof
 #define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER)
 #else
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 #endif
+#endif /* __KERNEL__ */
 
 #endif