asm-generic: don't include <asm/segment.h> from <asm/uaccess.h>

<asm/segment.h> is an odd x86 legacy that we shouldn't force on other
architectures.  arc used it to bring in mm_context_t, but we can do
that inside the arc code easily.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>