kismet: fix build when none of client, server or drone are selected

Random package configuration can trigger the selection of kismet, but
neither of client, server or drone suboptions. In this case, the
KISMET_TARGET_BINARIES variable is empty, leading to failure during
the installation stage.

At the same time, we remove the useless stripping of the installed
binaries, since this is done globally by the Buildroot infrastructure.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>