udftool: add missing dependency on readline

udftool/Config.in selects BR2_PACKAGE_READLINE, but udftool does not
depend on it. Readline is not a runtime dependency: one of the udftool
program links against it.

This commit fixes this inconsistency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>