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

Change-Id: I072c924ecdd212d63700804e8af3f4fac6da1919
Refs: #1681
diff --git a/src/security/validator-regex.hpp b/src/security/validator-regex.hpp
index 7c32e53..0490b77 100644
--- a/src/security/validator-regex.hpp
+++ b/src/security/validator-regex.hpp
@@ -47,6 +47,7 @@
 
   static const shared_ptr<CertificateCache> DEFAULT_CERTIFICATE_CACHE;
 
+  explicit
   ValidatorRegex(Face& face,
                  shared_ptr<CertificateCache> certificateCache = DEFAULT_CERTIFICATE_CACHE,
                  const int stepLimit = 3);