Revert "Minimize the changes in #5277"

This reverts commit dc51d89ac2.

src/variable.c

@@ -10,6 +10,7 @@
 #include <mruby/proc.h>
 #include <mruby/string.h>
 #include <mruby/variable.h>
+#include <mruby/presym.h>
 
 struct iv_elem {
   mrb_sym key;