powercap: Fix std::variant usage

Only mapbox has the .get() member function. Use the std::get compatible
function instead.

Change-Id: If132312404d1a0322889d6ad7bdb6b1cb399e075
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/powercap.cpp b/powercap.cpp
index bc4d3ca..638d5fe 100644
--- a/powercap.cpp
+++ b/powercap.cpp
@@ -1,3 +1,4 @@
+#include <cassert>
 #include <phosphor-logging/log.hpp>
 #include <powercap.hpp>
 
@@ -82,7 +83,7 @@
     sdbusplus::message::variant<uint32_t> pcap;
     reply.read(pcap);
 
-    return pcap.get<uint32_t>();
+    return sdbusplus::message::variant_ns::get<uint32_t>(pcap);
 }
 
 bool PowerCap::getPcapEnabled()
@@ -104,7 +105,7 @@
     sdbusplus::message::variant<bool> pcapEnabled;
     reply.read(pcapEnabled);
 
-    return pcapEnabled.get<bool>();
+    return sdbusplus::message::variant_ns::get<bool>(pcapEnabled);
 }
 
 void PowerCap::writeOcc(uint32_t pcapValue)