Squelch redundant declaration warnings.

Squelch redundant declaration warnings.

Have the text-to-pcap scanners define a routine that the main code
calls, which both allocates and destroys the scanner.  Don't declare the
Lex-generated routines in a header file we create, declare that routine,
instead.

Change-Id: Icad6a83db1a0dea8ac390315af72383fc99f8513
Reviewed-on: https://code.wireshark.org/review/25822
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>