AP_Proximity: add AP::proximity() singleton getter

libraries/AP_Proximity/AP_Proximity.cpp

@@ -465,3 +465,12 @@ bool AP_Proximity::sensor_failed() const
 }
 
 AP_Proximity *AP_Proximity::_singleton;
+
+namespace AP {
+
+AP_Proximity *proximity()
+{
+    return AP_Proximity::get_singleton();
+}
+
+}

libraries/AP_Proximity/AP_Proximity.h

@@ -161,3 +161,7 @@ private:
     void detect_instance(uint8_t instance);
     void update_instance(uint8_t instance);  
 };
+
+namespace AP {
+    AP_Proximity *proximity();
+};