Move ruby.h in `#ifdef RUBY` block

Must not depend on ruby specific files in Ruby unrelated cases.

regenc.h

@@ -40,6 +40,7 @@
 #   include "ruby/defines.h"
 #  endif
 # endif
+# include "ruby.h"
 #else /* RUBY */
 # ifndef PACKAGE
 /* PACKAGE is defined in config.h */
@@ -51,8 +52,6 @@
 # undef ONIG_ESCAPE_UCHAR_COLLISION
 #endif
 
-#include "ruby.h"
-
 #ifdef RUBY
 # include "ruby/onigmo.h"
 #else