patch 8.0.0375: the "+ register is not tested

Problem:    The "+ register is not tested.
Solution:   Add a test using another Vim instance to change the "+ register.
            (Kazuki Kuriyama)
diff --git a/src/version.c b/src/version.c
index b96daff..58ece75 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    375,
+/**/
     374,
 /**/
     373,