patch 8.0.1839: script to check .po file doesn't check for plural header

Problem:    Script to check .po file doesn't check for plural header.
Solution:   Add a check that the plural header is present when needed.

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1839,
 /**/
     1838,
 /**/