Fix the flex wrapper to ensure Cygwin /bin is on the path for Windows.
This is needed for CMake, as it doesn't add Cygwin to the path for the
generated solutions.

Change-Id: I9f05f24ccc741bfc851ecbb892f080f59d2acc2c
Reviewed-on: https://code.wireshark.org/review/2938
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Graham Bloice <graham.bloice@trihedral.com>