Add explicit keyword to 1 arg constructors

The cppcheck tool complains about it otherwise.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Id977a6c349b0b49354b52ac994ea7742e0a39307
diff --git a/control/json/manager.hpp b/control/json/manager.hpp
index 8bb5c35..c169919 100644
--- a/control/json/manager.hpp
+++ b/control/json/manager.hpp
@@ -162,7 +162,7 @@
      *
      * @param[in] event - sdeventplus event loop
      */
-    Manager(const sdeventplus::Event& event);
+    explicit Manager(const sdeventplus::Event& event);
 
     /**
      * @brief Callback function to handle receiving a HUP signal to reload the