esp_task_wdt.h doesn't have C++ calling convention (GIT8266O-437)

[awander]

When "components\esp8266\include\esp_task_wdt.h" is included in C++ code and 'esp_task_wdt_reset()' is called, it leads to a link error of "undefine reference to 'esp_task_wdt_reset()'" !

Insert following code should resolve this error:

#ifdef __cplusplus
extern "C" {
#endif

...

#ifdef __cplusplus
}
#endif