We have to include version_info.obj manually in the dependencies for text2pcap.

Add a comment indicating why we're not generating text2pcap_OBJECTS from
text2pcap_SOURCES and using that.

Change-Id: I8235080c3ea2bb31861a9c4f5aee9e6ce6a0808c
Reviewed-on: https://code.wireshark.org/review/2801
Reviewed-by: Guy Harris <guy@alum.mit.edu>