ui/vgafont: add SPDX license header

The vgafont was added without source origin, but it can be traced back
to Linux kernel, which used
GPL-2 (https://github.com/mpe/linux-fullhistory/blob/master/lib/fonts/font_8x16.c).

commit c6f37d0e4f
Author: Fabrice Bellard <fabrice@bellard.org>
Date:   Wed Jul 14 17:39:50 2004 +0000

    virtual console

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

ui/vgafont.c

@@ -1,3 +1,6 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
 #include "vgafont.h"
 
 const uint8_t vgafont16[256 * 16] = {

ui/vgafont.h

@@ -1,3 +1,6 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
 #ifndef VGAFONT_H
 #define VGAFONT_H