Remove unsupported C99 style struct initializers in git-archive.

At least one older version of the Solaris C compiler doesn't support
the newer C99 style struct initializers.  To allow Git to compile
on those systems use an archive description struct which is easier
to initialize without the C99 struct initializer syntax.

Also since the archives array is not used by anyone other than
archive.c we can make it static.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

archive.h

@@ -25,8 +25,6 @@ struct archiver {
 	parse_extra_args_fn_t parse_extra;
 };
 
-extern struct archiver archivers[];
-
 extern int parse_archive_args(int argc,
 			      const char **argv,
 			      struct archiver *ar);