Hardcode SensorValueType to double

A newer version of the compiler does not like how the sensor value type
was being determined from the D-Bus interface class, so just hardcode it
to double.  This was originally put in when the project was
transitioning from using an int64_t to a double for the type, but that's
been done for awhile now.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: If6a41e3ca43cf12cdebfb2137389b47a0d9dd947
diff --git a/interface.hpp b/interface.hpp
index 4b157dc..6fdfd51 100644
--- a/interface.hpp
+++ b/interface.hpp
@@ -29,10 +29,7 @@
     server::OperationalStatus;
 using StatusObject = ServerObject<StatusInterface>;
 
-// I understand this seems like magic, but since decltype doesn't evaluate you
-// can call nullptr https://stackoverflow.com/a/5580411/2784885
-using SensorValueType =
-    decltype((static_cast<ValueInterface*>(nullptr))->value());
+using SensorValueType = double;
 
 enum class InterfaceType
 {