Add GPL header to source files (wikitext, de37c4c)

Add GPL header to all source files, except those in the ANTLR subdirectory (ANTLR itself is licensed under the BSD license, and a copy of the ANTLR source archive is stored in/distributed with the repository for convenience and redundancy purposes only, as explained in the README).

Signed-off-by: Greg Hurrell <greg@hurrell.net>