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.
diff --git a/src/version.c b/src/version.c
index 04d7814..a9348ef 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1839,
+/**/
     1838,
 /**/
     1837,