zephyr: Bump heap size to 24K, as required for umqtt.simple.

zephyr/main.c

@@ -31,7 +31,7 @@ void do_str(const char *src, mp_parse_input_kind_t input_kind) {
 }
 
 static char *stack_top;
-static char heap[16 * 1024];
+static char heap[24 * 1024];
 
 int real_main(void) {
     int stack_dummy;