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: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>