Directly include <exception> for std::set_terminate (#8978)

I was recently trying to compile this project with a more recent version of GCC and hit an issue with this file. With older versions the std::set_terminate definition was being included via a transitive include of <exception> from the C++ standard library implementation. This is no longer the case with newer versions of libstdc++. This fixes the issue by using a direct include instead of relying on an implementation specific transitive include.

cores/esp8266/core_esp8266_main.cpp

@@ -24,6 +24,7 @@
 //#define CONT_STACKSIZE 4096
 
 #include <numeric>
+#include <exception>
 
 #include <Arduino.h>
 #include "Schedule.h"