Missing magic block

I'm trying to learn how to write custom extensions to postgres so wrote a
basic C function to see how it works. However, I keep getting the following
error "Missing magic block" when I try to add the function to the database.
According to the documentation in the manual, all I need to do is add the
following:

#include "server/fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

To my C file and it will work. However, I guess it is not working? Below
is the source code:

#include "server/postgres.h"
#include <string.h>

#include "server/fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

int
add_one(int arg)
{
return arg + 1;
}

And the sql statement I am using is:

CREATE FUNCTION add_one(IN int)
RETURNS int
AS 'add_one'
LANGUAGE C;

Any feedback as to how to correct it is appreciated!
Thanks,
Brad