Minimize the changes in #5277

Instead of including `mruby/presym.h` everywhere, we provided the
fallback `mruby/presym.inc` under `include/mruby` directory, and specify
`-I<build-dir>/include` before `-I<top-dir>/include` in `presym.rake`.
So even when someone drops `-I<build-dir>/include` in compiler options,
it just compiles without failure.

src/variable.c

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