include/linux/stddef.h: avoid 'warning: preprocessor token offsetof redefined'

hack to avoid:

/opt/gcc-4.6.3-nolibc/powerpc-linux/bin/../lib/gcc/powerpc-linux/4.6.3/include/stddef.h:413:9: warning: preprocessor token offsetof redefined
u-boot/include/linux/stddef.h:20:9: this was the original definition

tried protecting with __KERNEL__, and #including compiler.h first.

Even tried include dir reordering to no avail:

 +++ b/config.mk
 @@ -194,7 +194,11 @@ OBJCFLAGS += --gap-fill=0xff
 gccincdir := $(shell $(CC) -print-file-name=include)

 CPPFLAGS := $(DBGFLAGS) $(OPTFLAGS) $(RELFLAGS)                \
 -       -D__KERNEL__
 +       -D__KERNEL__  \
 +       -I $(gccincdir) \
 +       -nostdinc
 +
 +#      -isystem $(gccincdir) \

 # Enable garbage collection of un-used sections for SPL
 ifeq ($(CONFIG_SPL_BUILD),y)
 @@ -227,8 +231,7 @@ CPPFLAGS += -I$(OBJTREE)/include2 -I$(OBJTREE)/include
 endif

 CPPFLAGS += -I$(TOPDIR)/include
 -CPPFLAGS += -fno-builtin -ffreestanding -nostdinc      \
 -       -isystem $(gccincdir) -pipe $(PLATFORM_CPPFLAGS)
 +CPPFLAGS += -fno-builtin -ffreestanding -pipe $(PLATFORM_CPPFLAGS)

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>