uml: fix a compile error

Fix

arch/um/sys-i386/signal.c: In function 'copy_sc_from_user':
arch/um/sys-i386/signal.c:182: warning: dereferencing 'void *' pointer
arch/um/sys-i386/signal.c:182: error: request for member '_fxsr_env' in something not a structure or union

Signed-off-by: WANG Cong <wangcong@zeuux.org>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>