Fix subscription to wildcard topics

`mg_mqtt_match_topic_expression()` and friends are public now.

So far, only `#` is supported.

PUBLISHED_FROM=20c031bcf34be84771c4a9c456318634eb9a517c

docs/c-api/mqtt.h/mg_mqtt_vmatch_topic_expression.md

@@ -0,0 +1,11 @@
+---
+title: "mg_mqtt_vmatch_topic_expression()"
+decl_name: "mg_mqtt_vmatch_topic_expression"
+symbol_kind: "func"
+signature: |
+  int mg_mqtt_vmatch_topic_expression(const char *exp, struct mg_str topic);
+---
+
+Same as `mg_mqtt_match_topic_expression()`, but takes `exp` as a
+NULL-terminated string. 
+