package/gstreamer/zbar-plugin: propagate dependencies from zbar

BR2_PACKAGE_GST_PLUGINS_BAD_ZBAR select BR2_PACKAGE_ZBAR which
has several dependencies from libv4l.

Propagate these dependencies to avoid unmet dependencies while
selecting BR2_PACKAGE_GST_PLUGINS_BAD_ZBAR.

[Peter: show comment if toolchain dependencies aren't available]
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>