Add ICU ticket number for this patch

https://ssl.icu-project.org/trac/ticket/13175

Change-Id: Ib7756f3c41d2395f65d898e39808616c04ee58ee
diff --git a/external/icu/icu4c-59-werror-undef.patch.1 b/external/icu/icu4c-59-werror-undef.patch.1
index 196d789..0792e73 100644
--- a/external/icu/icu4c-59-werror-undef.patch.1
+++ b/external/icu/icu4c-59-werror-undef.patch.1
@@ -1,4 +1,5 @@
diff -ur icu.org/source/common/unicode/uvernum.h icu/source/common/unicode/uvernum.h
# https://ssl.icu-project.org/trac/ticket/13175
--- icu.org/source/common/unicode/uvernum.h	2017-03-21 02:03:49.000000000 +0100
+++ icu/source/common/unicode/uvernum.h	2017-04-21 23:44:55.123597927 +0200
@@ -125,7 +125,7 @@