CTestCustom: Ignore warnings in third-party zstd code

We already do this for other third-party libraries.

CTestCustom.cmake.in

@@ -17,6 +17,7 @@ list(APPEND CTEST_CUSTOM_WARNING_EXCEPTION
   "not sorted slower link editing will result"
   "stl_deque.h:479"
   "Utilities.cmzlib."
+  "Utilities.cmzstd."
   "Utilities.cmbzip2."
   "Source.CTest.Curl"
   "Source.CursesDialog.form"