um: os-linux/mem.c needs sys/stat.h

The os-linux/mem.c file calls fchmod function, which is declared in sys/stat.h
header file, so include it.  Fixes build breakage under FC13.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Acked-by: Boaz Harrosh <bharrosh@panasas.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>