Prevent compilation failure due to undefined Timer struct

Add a forward declaration for the implementation-dependant Timer struct to avoid build failures on the ESP32.

Signed-off-by: Denis Benato <[email protected]>

Author: NeroReflex

MQTTClient-C/src/MQTTClient.h

@@ -74,11 +74,13 @@ typedef struct Network
 
 /* The Timer structure must be defined in the platform specific header,
  * and have the following functions to operate on it.  */
-extern void TimerInit(Timer*);
-extern char TimerIsExpired(Timer*);
-extern void TimerCountdownMS(Timer*, unsigned int);
-extern void TimerCountdown(Timer*, unsigned int);
-extern int TimerLeftMS(Timer*);
+struct Timer;
+
+extern void TimerInit(struct Timer*);
+extern char TimerIsExpired(struct Timer*);
+extern void TimerCountdownMS(struct Timer*, unsigned int);
+extern void TimerCountdown(struct Timer*, unsigned int);
+extern int TimerLeftMS(struct Timer*);
 
 typedef struct MQTTMessage
 {