[master] Added explicit to IOServiceWork to remove 2 cppcheck regressions.

This is trivial fix which I am committing on my discretion.

src/bin/dhcp4/tests/ctrl_dhcp4_srv_unittest.cc

@@ -59,7 +59,7 @@ public:
     /// @brief Constructor.
     ///
     /// @param io_service Pointer to the IO service to be stopped.
-    IOServiceWork(const IOServicePtr& io_service)
+    explicit IOServiceWork(const IOServicePtr& io_service)
         : io_service_(io_service) {
     }
 

src/bin/dhcp6/tests/ctrl_dhcp6_srv_unittest.cc

@@ -56,7 +56,7 @@ public:
     /// @brief Constructor.
     ///
     /// @param io_service Pointer to the IO service to be stopped.
-    IOServiceWork(const IOServicePtr& io_service)
+    explicit IOServiceWork(const IOServicePtr& io_service)
         : io_service_(io_service) {
     }