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.

src/sasl.c

@@ -65,7 +65,7 @@ char *sasl_plain(xmpp_ctx_t *ctx, const char *authid, const char *password)
 /** helpers for digest auth */
 
 /* create a new, null-terminated string from a substring */
-static char *_make_string(xmpp_ctx_t *ctx, const char *s, const unsigned len)
+static char *_make_string(xmpp_ctx_t *ctx, const char *s, unsigned len)
 {
     char *result;