Add stdint, move headers up in core_features.h (esp8266#6793)

Fixes esp8266#6791 which I think only happened when the file was included from
a .C file (due to the headers being included only ifdef __cplusplus).

Author: earlephilhower

cores/esp8266/core_esp8266_features.h

@@ -32,10 +32,11 @@
 
 #define WIFI_HAS_EVENT_CALLBACK
 
-#ifdef __cplusplus
-
 #include <stdlib.h> // malloc()
 #include <stddef.h> // size_t
+#include <stdint.h>
+
+#ifdef __cplusplus
 
 namespace arduino
 {