Require explicit decorator on one arg constructors

We essentially follow this rule already, not relying on implicit
operators, although there are a number of cases where in theory we
could've implicitly constructed an object.

This commit enables the clang-tidy check.

Tested: Code compiles, passes clang-tidy.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ia428463313b075c69614fdb326e8c5c094e7adde
diff --git a/redfish-core/lib/certificate_service.hpp b/redfish-core/lib/certificate_service.hpp
index b931c7d..2fc606d 100644
--- a/redfish-core/lib/certificate_service.hpp
+++ b/redfish-core/lib/certificate_service.hpp
@@ -155,7 +155,7 @@
     CertificateFile& operator=(const CertificateFile&) = delete;
     CertificateFile(CertificateFile&&) = delete;
     CertificateFile& operator=(CertificateFile&&) = delete;
-    CertificateFile(const std::string& certString)
+    explicit CertificateFile(const std::string& certString)
     {
         std::array<char, 18> dirTemplate = {'/', 't', 'm', 'p', '/', 'C',
                                             'e', 'r', 't', 's', '.', 'X',