gdb/s390: Fix build breakage due to std::min/std::max usage without header

gdb/s390: Fix build breakage due to std::min/std::max usage without header

  [...]
  .../gdb/s390-linux-nat.c: In function 'void s390_prepare_to_resume(lwp_info*)':
  .../gdb/s390-linux-nat.c:703:20: error: 'min' is not a member of 'std'
      watch_lo_addr = std::min (watch_lo_addr, area->lo_addr);
  [...]

gdb/ChangeLog:
2016-09-18  Pedro Alves  <palves@redhat.com>

	* s390-linux-nat.c: Include <algorithm>.