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>

arch/um/sys-i386/signal.c

@@ -179,7 +179,8 @@ static int copy_sc_from_user(struct pt_regs *regs,
 	if (have_fpx_regs) {
 		struct user_fxsr_struct fpx;
 
-		err = copy_from_user(&fpx, &sc.fpstate->_fxsr_env[0],
+		err = copy_from_user(&fpx,
+			&((struct _fpstate __user *)sc.fpstate)->_fxsr_env[0],
 				     sizeof(struct user_fxsr_struct));
 		if (err)
 			return 1;