sensor: to enable testing receive hwmonio interface pointer

The sensor objects all share a reference to the mainloop's ioAccess
object.  To enable testing, the sensor object needs to expect the base
pointer to this object.

Change-Id: I1d7f2841528776c8d4f1166e20874ddeb4b8554a
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/sensor.hpp b/sensor.hpp
index 37cf035..3bcfc82 100644
--- a/sensor.hpp
+++ b/sensor.hpp
@@ -6,6 +6,7 @@
 
 #include <chrono>
 #include <gpioplus/handle.hpp>
+#include <memory>
 #include <unordered_set>
 
 namespace sensor
@@ -42,7 +43,7 @@
      * @param[in] devPath - Device sysfs path
      */
     explicit Sensor(const SensorSet::key_type& sensor,
-                    const hwmonio::HwmonIO& ioAccess,
+                    const hwmonio::HwmonIOInterface* ioAccess,
                     const std::string& devPath);
 
     /**
@@ -128,7 +129,7 @@
     SensorSet::key_type sensor;
 
     /** @brief Hwmon sysfs access. */
-    const hwmonio::HwmonIO& ioAccess;
+    const hwmonio::HwmonIOInterface* ioAccess;
 
     /** @brief Physical device sysfs path. */
     const std::string& devPath;