component/postfix: Add support for Linux 6.

  INFO (echo "# Do not edit -- this file documents how Postfix was built for your machine."; /bin/sh makedefs) >makedefs.tmp
  INFO ATTENTION:
  INFO ATTENTION: Unknown system type: Linux 6.1.6-arch1-3
  INFO ATTENTION:
  INFO make: *** [Makefile.in:33: Makefiles] Error 1
  INFO make: *** [Makefile:22: makefiles] Error 2