patch 8.0.0494: build failure with older compiler on MS-Windows

Problem:    Build failure with older compiler on MS-Windows.
Solution:   Move declaration to start of block.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 83ff48f..5dc7f82 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -8677,14 +8677,15 @@
     if (serverid != NULL && !check_restricted() && !check_secure())
     {
 	int timeout = 0;
+# ifdef WIN32
+	/* The server's HWND is encoded in the 'id' parameter */
+	long_u		n = 0;
+# endif
 
 	if (argvars[1].v_type != VAR_UNKNOWN)
 	    timeout = get_tv_number(&argvars[1]);
 
 # ifdef WIN32
-	/* The server's HWND is encoded in the 'id' parameter */
-	long_u		n = 0;
-
 	sscanf((char *)serverid, SCANF_HEX_LONG_U, &n);
 	if (n != 0)
 	    r = serverGetReply((HWND)n, FALSE, TRUE, TRUE, timeout);