test-string-list.c: Fix some sparse warnings

test-string-list.c: Fix some sparse warnings

In particular, sparse complains as follows:

        SP test-string-list.c
    test-string-list.c:10:6: warning: symbol 'parse_string_list' was not \
        declared. Should it be static?
    test-string-list.c:18:6: warning: symbol 'write_list' was not \
        declared. Should it be static?
    test-string-list.c:25:6: warning: symbol 'write_list_compact' was not \
        declared. Should it be static?
    test-string-list.c:38:5: warning: symbol 'prefix_cb' was not \
        declared. Should it be static?

In order to suppress the warnings, since the above symbols do not
need more than file scope, we simply include the static modifier
in their declaration.

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