Fix location of a ChangeLog entry file

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/undef_assert_before_defining_it.txt b/ChangeLog.d/undef_assert_before_defining_it.txt
similarity index 100%
rename from undef_assert_before_defining_it.txt
rename to ChangeLog.d/undef_assert_before_defining_it.txt