Updated Flex version from 2.6.0 to 2.6.4 and regenerated files.

Change-Id: I7bae30e1f0e1dc16de2bc27f908a1114449d756c
Reviewed-on: https://swiftshader-review.googlesource.com/16768Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>