[tlbuild] flex-2.6.0

[Karl Berry]

A new version of flex, 2.6.0, was released some time ago.  I (perhaps
foolishly, but as is our usual practice) have regenerated the
TL-maintained flex-derived sources and committed the new versions
(r40623).

Of course, this should make no difference to the behavior of anything.
But if any strange compilation problems suddenly crop up, this is a
likely culprit.  The files are:
  texk/detex/detex-src/detex.c
  texk/web2c/omegafonts/pl-lexer.c
  texk/web2c/otps/otp-lexer.c
  texk/web2c/web2c/web2c-lexer.c

Happy lexing,
Karl