patch 8.0.0840: MS-Windows: fopen() and open() prototypes are wrong

Problem:    MS-Windows: fopen() and open() prototypes do not match the ones in
            the system header file.  Can't build without FEAT_MBYTE.
Solution:   Add "const".  Move macro to after including protoo.h.

src/vim.h

@@ -2141,6 +2141,11 @@ typedef enum {
 # define number_width(x) 7
 #endif
 
+/* This must come after including proto.h */
+#if !(defined(FEAT_MBYTE) && defined(WIN3264))
+# define mch_open(n, m, p)	open((n), (m), (p))
+# define mch_fopen(n, p)	fopen((n), (p))
+#endif
 
 #include "globals.h"	    /* global variables and messages */