include: add missing headers

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I6b26a1bea2aae173b57dbd8affa0ba4991b1c9bb
diff --git a/include/PwmSensor.hpp b/include/PwmSensor.hpp
index 973d695..345e7b5 100644
--- a/include/PwmSensor.hpp
+++ b/include/PwmSensor.hpp
@@ -1,6 +1,8 @@
 #pragma once
 
+#include <memory>
 #include <sdbusplus/asio/object_server.hpp>
+#include <string>
 
 class PwmSensor
 {