src: add missing headers

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I6043ef792c2ce0a8e7584a73f83ca365ec61794d
diff --git a/src/PwmSensor.cpp b/src/PwmSensor.cpp
index 86a76f2..3fdde65 100644
--- a/src/PwmSensor.cpp
+++ b/src/PwmSensor.cpp
@@ -20,6 +20,8 @@
 #include <fstream>
 #include <iostream>
 #include <sdbusplus/asio/object_server.hpp>
+#include <stdexcept>
+#include <string>
 
 static constexpr size_t pwmMax = 255;
 static constexpr double defaultPwm = 30.0;