Declare "yylex()" in "text2pcap.h", so that there's a prototype in scope

Declare "yylex()" in "text2pcap.h", so that there's a prototype in scope
when it's used by "text2pcap.c" - and so that the *same* prototype is in
scope when it's defined by "text2pcap-scanner.c".

Define "YY_NO_UNPUT" in "text2pcap.h", so we don't define "yyunput()" in
"text2pcap-scanner.c"; it's not used, so some compilers whine about it.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@3424 f5534014-38df-0310-8fa8-9805f1628bb7