Use forward slashes in paths to runlex.sh

This gets rid of "MS-DOS style path detected" warnings from Cygwin.

Change-Id: Id10429669704aa371dbf56a9398947c8002260ad
Reviewed-on: https://code.wireshark.org/review/3024
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Wireshark Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>