patch 8.0.1604: paste test may fail if $DISPLAY is not set

Problem:    Paste test may fail if $DISPLAY is not set.
Solution:   Add WorkingClipboard() and use it in the paste test.
diff --git a/src/version.c b/src/version.c
index d1fd6ea..36e94b1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1604,
+/**/
     1603,
 /**/
     1602,