dbus: fix a hard dependency about dbus-ptest

dbus: fix a hard dependency about dbus-ptest

If image contains dbus and ptest is in DISTRO_FEATURES, dbus-ptest package
is installed, regardless of whether ptest-pkgs is in IMAGE_FEATURES. This
issue will increase size for most small images.
This patch fixes this problem.

[YOCTO #5702]

(From OE-Core rev: 0416583f014138656babdf78a574357ae5ff25bd)

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>