Mrbgem gem_init.c compilation correction

When `disable_cdump` is declared on a Mrbgem spec, the procedure for
loading MRuby code from the gem's `mrblib` directory is slightly
different; instead of loading the mrblib code as a Proc built from the
compiled irep, the compiled irep is loaded directly. The header files
`mruby.h` and `mruby/proc.h` are needed only when the irep is loaded
directly. They are currently included only when presym is disabled, they
should be included whenever either presym is disabled *or* when
`disable_cdump` is called. The `cdump?` predicate method happens to
return true in either case (presym disabled *or* cdump disabled), so
this change should be safe.

lib/mruby/gem.rb

@@ -253,7 +253,7 @@ module MRuby
           f.puts %Q[#include <mruby.h>]
         else
           f.puts %Q[#include <stdlib.h>]
-          unless build.presym_enabled?
+          unless cdump?
             f.puts %Q[#include <mruby.h>]
             f.puts %Q[#include <mruby/proc.h>]
           end