psu-ng: Implement the PowerSystemInputs interface

Implement the PowerSystemInputs interface which contains the status of
the power inputs to the chassis. The psu monitor app will use the status
property to communicate if a brownout condition exists.
Implement this interface under a /chassis0 path to specify it applies to
the current chassis. This can later be enhanced to support a
multi-chassis configuration.

Tested: The status property exists and it's set to default value Good:
root@p10bmc:~# busctl get-property xyz.openbmc_project.Power.PSUMonitor\
    /xyz/openbmc_project/power/power_supplies/chassis0/psus\
    xyz.openbmc_project.State.Decorator.PowerSystemInputs Status
s "xyz.openbmc_project.State.Decorator.PowerSystemInputs.Status.Good"

Change-Id: I7cf4d13e632841e0b19b52af0d9eac886eb161ce
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/phosphor-power-supply/meson.build b/phosphor-power-supply/meson.build
index f806861..bb90aea 100644
--- a/phosphor-power-supply/meson.build
+++ b/phosphor-power-supply/meson.build
@@ -13,6 +13,7 @@
         sdeventplus,
         fmt,
         libgpiodcxx,
+        phosphor_dbus_interfaces,
     ],
     include_directories: '..',
     install: true,