rbtree: avoid redefining offsetof

rbtree: avoid redefining offsetof

Newer versions of gcc will define & export offsetof, so we don't
want to unconditionally define it.  Otherwise we hit:
	In file included from mkfs.jffs2.c:76:0:
	rbtree.h:134:0: warning: "offsetof" redefined
	/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.1/include/stddef.h:411:0: note:
		this is the location of the previous definition

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>