builtin/tag.c: Fix a sparse warning

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>