patch 8.0.0950: MS-Windows: wrong #ifdef, compiler warnings

Problem:    MS-Windows: wrong #ifdef, compiler warnings for signed/unsigned.
Solution:   Change variable type. Change TERMINAL to FEAT_TERMINAL.
diff --git a/src/os_win32.c b/src/os_win32.c
index 1098d7f..f1e1cad 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -5072,7 +5072,7 @@
 	    --todo;
 	    if (wkey != NULL && wval != NULL)
 	    {
-		int	n;
+		size_t	n;
 		size_t	lkey = wcslen(wkey);
 		size_t	lval = wcslen(wval);