patch 9.0.0918: MS-Windows: modifier keys do not work with mouse scroll event

Problem:    MS-Windows: modifier keys do not work with mouse scroll events.
Solution:   Use K_SPECIAL instead of CSI for the modifier keys. (Christopher
            Plewright, closes #11587)
diff --git a/src/os_win32.c b/src/os_win32.c
index 36051f4..ead50e1 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -2047,7 +2047,8 @@
 	    {
 		if (modifiers > 0)
 		{
-		    typeahead[typeaheadlen++] = CSI;
+		    // use K_SPECIAL instead of CSI to make mappings work
+		    typeahead[typeaheadlen++] = K_SPECIAL;
 		    typeahead[typeaheadlen++] = KS_MODIFIER;
 		    typeahead[typeaheadlen++] = modifiers;
 		}