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/string.c

@@ -22,7 +22,6 @@
 #include <mruby/range.h>
 #include <mruby/string.h>
 #include <mruby/numeric.h>
-#include <mruby/presym.h>
 
 typedef struct mrb_shared_string {
   int refcnt;