dev-manual: gen-tapdevs need iptables installed

Add the hint to the test setup that runqemu-gen-tapdevs will need the
iptables package installed.

(From yocto-docs rev: f703ac90d4fe6f004997e963c0cfc98c223bc146)

Signed-off-by: Simone Weiß <simone.p.weiss@posteo.com>
Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/dev-manual/dev-manual-common-tasks.rst

@@ -8652,6 +8652,8 @@ In order to run tests, you need to do the following:
          -  Be sure to use an absolute path when calling this script
             with sudo.
 
+         -  Ensure that your host has the package ``iptables`` installed.
+
          -  The package recipe ``qemu-helper-native`` is required to run
             this script. Build the package using the following command:
             ::