Merge branch 'export_global_init' of https://github.com/CallumAttryde/paho.mqtt.c into develop

PR #310

src/MQTTAsync.h

@@ -206,7 +206,7 @@ typedef struct
  * Global init of mqtt library. Call once on program start to set global behaviour.
  * handle_openssl_init - if mqtt library should handle openssl init (1) or rely on the caller to init it before using mqtt (0)
  */
-void MQTTAsync_global_init(MQTTAsync_init_options* inits);
+DLLExport void MQTTAsync_global_init(MQTTAsync_init_options* inits);
 
 /**
  * A handle representing an MQTT client. A valid client handle is available