dbus-test: make do_install run to trigger the effect of cleandirs flag

dbus-test: make do_install run to trigger the effect of cleandirs flag

For now, we would meet the following QA error if we build dbus-test in
the same build directory with multilib enabled/disabled.

The steps are as follows:
1. Enable multilib and `bitbake dbus-ptest'
2. Disable multilib and `bitbake dbus-ptest'

The error message is as follows:
ERROR: QA Issue: dbus-test: Files/directories were installed but not shipped
  /usr/lib64
  /usr/lib64/dbus-test
  /usr/lib64/dbus-test/ptest
  /usr/lib64/dbus-test/ptest/run-ptest
  ...

The problem is that the ${D} is not cleaned up at the second build while it
should be.

This patch fixes the above problem.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>