Merge pull request #12299 from pokgak/pr/credman_change_to_const_void

credman: change credman_buffer_t::s type to 'const void *'

sys/include/net/credman.h

@@ -45,7 +45,7 @@ extern "C" {
  * @brief Buffer of the credential
  */
 typedef struct {
-    void *s;                /**< Pointer to the buffer */
+    const void *s;          /**< Pointer to the buffer */
     size_t len;             /**< Length of credman_buffer_t::s */
 } credman_buffer_t;