patch 8.1.2257: MS-Windows GUI: scroll wheel always uses current window

Problem:    MS-Windows GUI: scroll wheel always uses current window.
Solution:   Add the 'scrollfocus' option for MS-Windows.
diff --git a/src/option.h b/src/option.h
index 3ee9979..1154aa8 100644
--- a/src/option.h
+++ b/src/option.h
@@ -814,6 +814,9 @@
 EXTERN char_u	*p_pp;		// 'packpath'
 EXTERN char_u	*p_rtp;		// 'runtimepath'
 EXTERN long	p_sj;		// 'scrolljump'
+#if defined(MSWIN) && defined(FEAT_GUI)
+EXTERN int	p_scf;		// 'scrollfocus'
+#endif
 EXTERN long	p_so;		// 'scrolloff'
 EXTERN char_u	*p_sbo;		// 'scrollopt'
 EXTERN char_u	*p_sections;	// 'sections'