Export mrb_int_value that converts mrb_float to Fixnum.

Or `Float` if `mrb_float` value is too big (or too small) to fit in
`mrb_int`. The `_int_` in `mrb_int_value` means `Integral` module, which
represents integer-like values in mruby.

src/numeric.c

@@ -45,7 +45,7 @@ mrb_to_flo(mrb_state *mrb, mrb_value val)
   return mrb_float(val);
 }
 
-static mrb_value
+MRB_API mrb_value
 mrb_int_value(mrb_state *mrb, mrb_float f)
 {
   if (FIXABLE_FLOAT(f)) {