patch 8.1.0480: MinGW build file uses different -I flags than MVC

Problem:    MinGW build file uses different -I flags than MVC.
Solution:   Add -I to $CFLAGS. (Ken takata)
diff --git a/src/version.c b/src/version.c
index 350c8a7..1eab778 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    480,
+/**/
     479,
 /**/
     478,