LEX and YACC are required

When building both LEX (flex) and YACC (bison or yacc) are required to generate C files from l(ex) and y(acc) files

Change-Id: I8892a05eb437d0bef613b68f681225df2cb2c6b4
Reviewed-on: https://code.wireshark.org/review/5515
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>