Add the missing definition for `as_entry`

connectivity/FEATURE_BLE/source/generic/KVStoreSecurityDb.h

@@ -40,6 +40,8 @@ private:
         sign_count_t peer_sign_counter;
     };
 
+    static entry_t *as_entry(entry_handle_t db_handle);
+    
     static constexpr uint8_t KVSTORESECURITYDB_VERSION = 1;
 
     static constexpr size_t DB_PREFIX_SIZE = 7 + sizeof (STR(MBED_CONF_STORAGE_DEFAULT_KV)) - 1;