include ruby/ruby.h before ruby.io.h

It breaks AIX build:
https://rubyci.org/logs/rubyci.s3.amazonaws.com/aix71_ppc/ruby-trunk/log/20180117T113303Z.log.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61893 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: nurse

marshal.c

@@ -13,6 +13,7 @@
 # error too old GCC
 #endif
 
+#include "ruby/ruby.h"
 #include "ruby/io.h"
 #include "internal.h"
 #include "ruby/st.h"