git-compat-util.h: don't define _XOPEN_SOURCE on cygwin

diff --git a/RelNotes b/RelNotes
index 2bff7b20de8972e2a8cf9bf7a8cea4ff2e9789f1..192f48cc9470b11e01edda4f0a84a257480ca96c 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.9.4.txt
\ No newline at end of file
+Documentation/RelNotes/2.1.3.txt
\ No newline at end of file