Emit deprecation warnings for rb_iterate()

* Already in 2.7 for TruffleRuby (3.1 for CRuby, but the function is
  marked as obsolete since 1.9) because rb_funcall() no longer
  magically pass the block when under rb_iterate() on TruffleRuby.
* Like ruby/ruby#4629

Author: eregon

lib/cext/ABI_check.txt

@@ -1 +1 @@
-5
+6

lib/cext/include/ruby/ruby.h

@@ -2207,6 +2207,7 @@ int rb_keyword_given_p(void);
 int rb_block_given_p(void);
 void rb_need_block(void);
 VALUE rb_iterate(VALUE(*)(VALUE),VALUE,rb_block_call_func_t,VALUE);
+DEPRECATED_BY(rb_block_call since 1.9, VALUE rb_iterate(VALUE(*)(VALUE),VALUE,rb_block_call_func_t,VALUE));
 VALUE rb_block_call(VALUE,ID,int,const VALUE*,rb_block_call_func_t,VALUE);
 VALUE rb_block_call_kw(VALUE,ID,int,const VALUE*,rb_block_call_func_t,VALUE,int);
 VALUE rb_rescue(VALUE(*)(VALUE),VALUE,VALUE(*)(VALUE,VALUE),VALUE);