security: Make addKey/Cert behavior of PibImpl consistent

Change-Id: I30c6dd0007e9095ee07f7a8eda3b20db4c34c513
Refs: #3351
diff --git a/src/security/v2/certificate.hpp b/src/security/v2/certificate.hpp
index 4e1b0eb..c451277 100644
--- a/src/security/v2/certificate.hpp
+++ b/src/security/v2/certificate.hpp
@@ -132,7 +132,7 @@
    * @brief Get public key bits (in PKCS#8 format)
    * @throw Error If content is empty
    */
-  const Buffer
+  Buffer
   getPublicKey() const;
 
   /**