error.h: add extern "C" for C++

include/mruby/error.h

@@ -7,6 +7,10 @@
 #ifndef MRUBY_ERROR_H
 #define MRUBY_ERROR_H
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 void mrb_sys_fail(mrb_state *mrb, const char *mesg);
 mrb_value mrb_exc_new_str(mrb_state *mrb, struct RClass* c, mrb_value str);
 #define mrb_exc_new_str_lit(mrb, c, lit) mrb_exc_new_str(mrb, c, mrb_str_new_lit(mrb, (lit)))
@@ -17,4 +21,8 @@ void mrb_print_backtrace(mrb_state *mrb);
 mrb_value mrb_exc_backtrace(mrb_state *mrb, mrb_value exc);
 mrb_value mrb_get_backtrace(mrb_state *mrb);
 
+#if defined(__cplusplus)
+}  /* extern "C" { */
+#endif
+
 #endif  /* MRUBY_ERROR_H */