vga: Fix warning caused by missing 'static' attribute

Warning from the Sparse static analysis tool:

hw/display/vga.c:2012:26: warning:
 symbol 'vmstate_vga_endian' was not declared. Should it be static?
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/display/vga.c

@@ -2032,7 +2032,7 @@ static bool vga_endian_state_needed(void *opaque)
     return s->default_endian_fb != s->big_endian_fb;
 }
 
-const VMStateDescription vmstate_vga_endian = {
+static const VMStateDescription vmstate_vga_endian = {
     .name = "vga.endian",
     .version_id = 1,
     .minimum_version_id = 1,