docs/manual/prerequisite.txt: add findutils in dependencies

The "find" and "xargs" commands, from the "findutils" package are used
during the build process. See for example [1].

Even if it's a quite common package which is almost sure to be present
on the host, it should be listed here. When writing new recipes, hooks
and scripts, it is generally safe and portable to restrict to the
host dependencies listed in those prerequisites.

This commit just add the missing "findutils" package in this list.

[1] https://git.buildroot.org/buildroot/tree/Makefile?h=2022.05.1#n737

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>