refactor: add [[noreturn]] attribute to execute_child (#132)

Author: fanquake

cpp-subprocess/subprocess.hpp

@@ -1183,7 +1183,7 @@ class Child
     err_wr_pipe_(err_wr_pipe)
   {}
 
-  void execute_child();
+  [[noreturn]] void execute_child();
 
 private:
   // Lets call it parent even though