builtin/tag.c: Fix a sparse warning

In particular, sparse complains as follows:

        SP builtin/tag.c
    builtin/tag.c:411:5: warning: symbol 'parse_opt_points_at' was \
        not declared. Should it be static?

In order to suppress the warning, since the parse_opt_points_at()
function does not need to be an external symbol, we simply add the
static modifier to the function definition.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -400,7 +400,7 @@ static int strbuf_check_tag_ref(struct strbuf *sb, const char *name)
 	return check_refname_format(sb->buf, 0);
 }
 
-int parse_opt_points_at(const struct option *opt __attribute__ ((unused)),
+static int parse_opt_points_at(const struct option *opt __attribute__((unused)),
 			const char *arg, int unset)
 {
 	unsigned char sha1[20];