add_layer_dependencies() might get called more than once, or one of
the layer dependencies might already be present. The function should
not add layers again because doing so can cause warnings like:

  WARNING: Duplicate inclusion for .../meta-openembedded/meta-oe/conf/distro/include/meta_oe_security_flags.inc in .../meta-openembedded/meta-oe/conf/layer.conf

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
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