include: add missing headers

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I6b26a1bea2aae173b57dbd8affa0ba4991b1c9bb
diff --git a/include/IpmbSensor.hpp b/include/IpmbSensor.hpp
index e7a9540..f226483 100644
--- a/include/IpmbSensor.hpp
+++ b/include/IpmbSensor.hpp
@@ -5,6 +5,9 @@
 #include <boost/container/flat_map.hpp>
 #include <chrono>
 #include <limits>
+#include <memory>
+#include <optional>
+#include <string>
 #include <vector>
 
 enum class IpmbType