fix the following 'make includecheck' warning:

  arch/s390/kernel/sys_s390.c: linux/syscalls.h is included more than once.
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: schwidefsky@de.ibm.com
Cc: heiko.carstens@de.ibm.com
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <1247068809.4382.111.camel@ht.satnam>