fix: don't remove VTERM_ANSI_INDEX_NONE

Author: izik1

src/term.c

@@ -3901,6 +3901,8 @@ static int grey_ramp[] = {
     0x08, 0x12, 0x1C, 0x26, 0x30, 0x3A, 0x44, 0x4E, 0x58, 0x62, 0x6C, 0x76,
     0x80, 0x8A, 0x94, 0x9E, 0xA8, 0xB2, 0xBC, 0xC6, 0xD0, 0xDA, 0xE4, 0xEE};
 
+#define VTERM_ANSI_INDEX_NONE 0
+
 static char_u ansi_table[16][4] = {
     //   R    G    B   idx
     {0, 0, 0, 1},       // black