Change exit(0) to sys.exit(0) in kconfig_xml_fixup.py

If pass -S option in python binaries wrapper, kconfig_xml_fixup
will meet below error (NameError: name 'exit' is not defined).

Bug:122343420
Test: 1. full build.
      2. m kconfig_xml_fixup
      out/soong/host/linux-x86/bin/kconfig_xml_fixup
      --input kernel/configs/android-4.14/android-base-conditional.xml
      --output-version
      out/soong/.intermediates/kernel/configs/android-4.14/kernel_config_current_4.14/gen/version.txt
      --output-matrix
      out/soong/.intermediates/kernel/configs/android-4.14/kernel_config_current_4.14/gen/conditional.xml

Change-Id: I1fef27321c1051a7a6e9e809328c0695f83cfced