Stick with one version of pcap_dump_ftell.c.

Stick with one version of pcap_dump_ftell.c.

It looks like CMake after commit 3e9e2b6 started to use the newly added
missing/pcap_dump_ftell.c to make pcap_dump_ftell() available in tcpdump
if libpcap does not have it. However, autotools continued to use the
previously existing ./pcap_dump_ftell.c for the same purpose. Remove the
previously existing file and amend autotools files to cover
pcap_dump_ftell() the same way as the other functions in the missing/
directory files.

Amend missing/pcap_dump_ftell.c not to use pcap_dump_file(), as it may be
unavailable.

This has been tested to work with libpcap 0.6.1.