t5550: add ICONV prereq to tests that use "$HTTPD_URL/error"

t5550: add ICONV prereq to tests that use "$HTTPD_URL/error"

We've got a bunch of tests in t5550 that connect to "$HTTPD_URL/error"
to ensure that error messages are properly forwarded. This URL executes
the "t/lib-httpd/error.sh" script, which in turn depends on the iconv(1)
executable to reencode the message.

This executable may not exist on platforms, which will make the tests
fail. Guard them with the ICONV prereq to fix such failures.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>