Fix std::variant usage

We need to conform to the std::variant interface for sdbusplus to
eventually switch away from mapbox. The .get() interface is only
provided by mapbox and needs to be converted.

Change-Id: Ia41808d6bab73b097fc631fd9eff549be9a22415
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/fault-monitor/fru-fault-monitor.cpp b/fault-monitor/fru-fault-monitor.cpp
index a7ed952..ca31702 100644
--- a/fault-monitor/fru-fault-monitor.cpp
+++ b/fault-monitor/fru-fault-monitor.cpp
@@ -282,7 +282,8 @@
                 entry("REPLY_SIG=%s", reply.get_signature()));
             continue;
         }
-        auto& assocs = assoc.get<AssociationList>();
+        auto& assocs =
+            sdbusplus::message::variant_ns::get<AssociationList>(assoc);
         if (assocs.empty())
         {
             // no associations, skip