Fix implicit declaration of rb_str_change_terminator_length()
Upstream-Status: Backport (from Ruby 2.3+)
--- internal.h.orig
+++ internal.h
@@ -988,6 +988,7 @@ VALUE rb_id_quote_unprintable(ID);
 #define QUOTE(str) rb_str_quote_unprintable(str)
 #define QUOTE_ID(id) rb_id_quote_unprintable(id)
 void rb_str_fill_terminator(VALUE str, const int termlen);
+void rb_str_change_terminator_length(VALUE str, const int oldtermlen, const int termlen);
 VALUE rb_str_locktmp_ensure(VALUE str, VALUE (*func)(VALUE), VALUE arg);
 #ifdef RUBY_ENCODING_H
 VALUE rb_external_str_with_enc(VALUE str, rb_encoding *eenc);