Add missing time.h include, use relative include path on time.h includes (#6730)

* Add missing time.h include

* Change other includes of time.h as well

* Added comment

Author: Niek

cores/esp8266/FS.h

@@ -23,6 +23,7 @@
 
 #include <memory>
 #include <Arduino.h>
+#include <../include/time.h> // See issue #6714
 
 class SDClass;
 

cores/esp8266/HardwareSerial.h

@@ -28,7 +28,7 @@
 #define HardwareSerial_h
 
 #include <inttypes.h>
-#include <time.h>
+#include <../include/time.h> // See issue #6714
 #include "Stream.h"
 #include "uart.h"
 

cores/esp8266/libc_replacements.cpp

@@ -31,7 +31,7 @@
 #include <limits.h>
 #include <errno.h>
 #include <string.h>
-#include <time.h>
+#include <../include/time.h> // See issue #6714
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>

cores/esp8266/time.cpp

@@ -17,7 +17,7 @@
  */
 
 #include <stdlib.h>
-#include <time.h>
+#include <../include/time.h> // See issue #6714
 #include <sys/time.h>
 #include <sys/reent.h>
 #include "sntp.h"