security: Add "explicit" markers for one-parameter constructors

Change-Id: I072c924ecdd212d63700804e8af3f4fac6da1919
Refs: #1681
diff --git a/src/security/conf/checker.hpp b/src/security/conf/checker.hpp
index 12017e2..a9ab2a9 100644
--- a/src/security/conf/checker.hpp
+++ b/src/security/conf/checker.hpp
@@ -193,6 +193,7 @@
 class HierarchicalChecker : public CustomizedChecker
 {
 public:
+  explicit
   HierarchicalChecker(uint32_t sigType)
     : CustomizedChecker(sigType,
         make_shared<HyperKeyLocatorNameChecker>("^(<>*)$", "\\1",