Extern C blocks (#1352) (#4044)

* Add extern C guard blocks to SDK header files #1352

* fixed some extern C blocks in core and libraries

tools/sdk/include/sntp.h

@@ -7,6 +7,11 @@
 #else
 #include "ip_addr.h"
 #endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * get the seconds since Jan 01, 1970, 00:00 (GMT + 8)
  */
@@ -65,4 +70,9 @@ char *sntp_getservername(unsigned char idx);
 
 #define sntp_servermode_dhcp(x)
 
+
+#ifdef __cplusplus
+}
+#endif
+
 #endif