fix build errors w/newer kernel headers & glibc

Building with linux-headers-3.9 and glibc-2.17 fails like so:
In file included from summary.h:15:0,
                 from jffs2dump.c:37:
/usr/include/linux/uio.h:16:8: error: redefinition of 'struct iovec'
 struct iovec
        ^
In file included from /usr/include/bits/fcntl-linux.h:38:0,
                 from /usr/include/bits/fcntl.h:61,
                 from /usr/include/fcntl.h:35,
                 from jffs2dump.c:25:
/usr/include/bits/uio.h:43:8: note: originally defined here
 struct iovec
        ^

Signed-off-by: Mike Frysinger <vapier@gentoo.org>