patch 8.0.1783: cannot use 256 colors in a MS-Windows console

Problem:    Cannot use 256 colors in a MS-Windows console.
Solution:   Add 256 color support. (Nobuhiro Takasaki, closes #2821)
diff --git a/src/proto/os_win32.pro b/src/proto/os_win32.pro
index 1bed98a..7f45c5c 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -72,4 +72,5 @@
 void control_console_color_rgb(void);
 int has_vtp_working(void);
 int use_vtp(void);
+int is_term_win32(void);
 /* vim: set ft=c : */