Add missing PR mention in ChangeLog

I noticed that the patch pushed previously had an open bug about it, so
add a reference to it.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e15e9f2..c4ad2bf 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -35,6 +35,7 @@
 
 2017-08-07  Leszek Swirski  <leszeks@google.com>
 
+       PR symtab/20899
        * dwarf2read.c (dwarf2_string_attr): Allow DW_FORM_GNU_strp_alt.
 
 2017-08-07  Simon Marchi  <simon.marchi@ericsson.com>