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.

tests/test_rand.c

@@ -21,14 +21,14 @@
 #include "rand.c"
 
 /* stubs to build test without whole libstrophe */
-void *xmpp_alloc(const xmpp_ctx_t *const ctx, const size_t size)
+void *xmpp_alloc(const xmpp_ctx_t *ctx, size_t size)
 {
     (void)ctx;
     (void)size;
     return NULL;
 }
 
-void xmpp_free(const xmpp_ctx_t *const ctx, void *p)
+void xmpp_free(const xmpp_ctx_t *ctx, void *p)
 {
     (void)ctx;
     (void)p;