style: remove extra const keyword from interfaces

Const variables in prototypes don't add much value, but make the code
larger and redundant. Remove these const keywords.
Note, this doesn't apply to pointers to const memory.

examples/basic.c

@@ -19,11 +19,11 @@
 #define KA_INTERVAL 1
 
 /* define a handler for connection events */
-void conn_handler(xmpp_conn_t *const conn,
-                  const xmpp_conn_event_t status,
-                  const int error,
-                  xmpp_stream_error_t *const stream_error,
-                  void *const userdata)
+void conn_handler(xmpp_conn_t *conn,
+                  xmpp_conn_event_t status,
+                  int error,
+                  xmpp_stream_error_t *stream_error,
+                  void *userdata)
 {
     xmpp_ctx_t *ctx = (xmpp_ctx_t *)userdata;
     int secured;