patch 8.1.0306: plural messages are not translated properly

Problem:    Plural messages are not translated properly.
Solution:   Add more usage of NGETTEXT(). (Sergey Alyoshin)
diff --git a/src/version.c b/src/version.c
index 9cb1041..5f03975 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    306,
+/**/
     305,
 /**/
     304,