Fix compilation errors for MQTT gateway

drivers/Linux/Arduino.h

@@ -51,6 +51,7 @@ char *ltoa(long value, char* result, int base);
 char *ultoa(long num, char *str, int radix);
 char *utoa(int num, char *str, int radix);
 char *dtostrf(float f, int width, int decimals, char *result);
+unsigned long millis(void);
 void delay(unsigned int millis);
 
 #ifdef __cplusplus

drivers/Linux/compatibility.cpp

@@ -1,7 +1,11 @@
 #include <stdio.h>
 #include <time.h>
+#include <sys/time.h>
 #include "Arduino.h"
 
+// For millis()
+static unsigned long millis_at_start = 0;
+
 /**
 * C++ version 0.4 char* style "itoa":
 * Written by Lukás Chmela
@@ -177,6 +181,19 @@ char *dtostrf(float f, int width, int decimals, char *result)
 	return result;
 }
 
+unsigned long millis(void)
+{
+	timeval curTime;
+
+	if (millis_at_start == 0) {
+		gettimeofday(&curTime, NULL);
+		millis_at_start = curTime.tv_sec;
+	}
+
+	gettimeofday(&curTime, NULL);
+	return ((curTime.tv_sec - millis_at_start) * 1000) + (curTime.tv_usec / 1000);
+}
+
 void delay(unsigned int millis)
 {
     struct timespec sleeper;