Fix a typing error in a conditional compiling directive
    (reported by Vasil Dimov: Thanks !),
    and improve their nesting ("MAIN" independent of platform).


mysys/my_gethwaddr.c:
  Fix a typing error, reported by Vasil Dimov (thanks !):
  The condition is meant to apply to FreeBSD or Linux.
  
  Improve the nesting of conditional compiling directives,
  handle "MAIN" and the platform issues independent of each other.