patch 8.0.1785: missing symbol in Win32 small build

Problem:    Missing symbol in Win32 small build.
Solution:   Define VTERM_ANSI_INDEX_NONE without the terminal feature. Also
            fix unused function with #ifdef.
diff --git a/src/term.c b/src/term.c
index d2a6e45..08eee67 100644
--- a/src/term.c
+++ b/src/term.c
@@ -6977,6 +6977,8 @@
 
 # ifdef FEAT_TERMINAL
 #  include "libvterm/include/vterm.h"  // for VTERM_ANSI_INDEX_NONE
+# else
+#  define VTERM_ANSI_INDEX_NONE 0
 # endif
 
 static uint8_t ansi_table[16][4] = {