Fix std::variant usage

std::variant only provides an std::get() implementation and not the
mapbox specific .get() member interface.

Change-Id: Icb95337b014329c1eb365310d220ec893b8afc58
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/dbus_watch.cpp b/src/dbus_watch.cpp
index fa1bcb1..ca5118e 100644
--- a/src/dbus_watch.cpp
+++ b/src/dbus_watch.cpp
@@ -139,7 +139,7 @@
     if (itc != conds_.end()) {
         const auto itp = properties.find(itc->second.property);
         if (itp != properties.end()) {
-            const auto& propVal = itp->second.get<std::string>();
+            const auto& propVal = sdbusplus::message::variant_ns::get<std::string>(itp->second);
             needFlush = itc->second.values.find(propVal) != itc->second.values.end();
         }
     }