Add prototype of rb_ary_ptr_use_start()

Without this icc fails to run.
http://rubyci.s3.amazonaws.com/icc-x64/ruby-trunk/log/20181203T130002Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66200 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

internal.h

@@ -1296,6 +1296,7 @@ VALUE rb_to_array_type(VALUE obj);
 VALUE rb_check_to_array(VALUE ary);
 VALUE rb_ary_tmp_new_from_values(VALUE, long, const VALUE *);
 VALUE rb_ary_behead(VALUE, long);
+void rb_ary_detransient(VALUE a);
 #if defined(__GNUC__) && defined(HAVE_VA_ARGS_MACRO)
 #define rb_ary_new_from_args(n, ...) \
     __extension__ ({ \