patch 8.0.0456: typo in MinGW test makefile

Problem:    Typo in MinGW test makefile.
Solution:   Change an underscore to a dot. (Michael Soyka)
diff --git a/src/version.c b/src/version.c
index 4f0066b..56c66b5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    456,
+/**/
     455,
 /**/
     454,