um: Add uaccess.h to syscalls_64.c

It's using put_user() and needs this header.
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/syscalls_64.c

@@ -6,6 +6,7 @@
  */
 
 #include <linux/sched.h>
+#include <linux/uaccess.h>
 #include <asm/prctl.h> /* XXX This should get the constants from libc */
 #include <os.h>