arch: c6x: mm: include "asm/uaccess.h" to pass compiling

Need include "asm/uaccess.h" to pass compiling.

The related error (with allmodconfig):

  arch/c6x/mm/init.c: In function `paging_init':
  arch/c6x/mm/init.c:46:2: error: implicit declaration of function `set_fs' [-Werror=implicit-function-declaration]
  arch/c6x/mm/init.c:46:9: error: `KERNEL_DS' undeclared (first use in this function)
  arch/c6x/mm/init.c:46:9: note: each undeclared identifier is reported only once for each function it appears in

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: <stable@vger.kernel.org>	[3.10.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>