console/vtxprintf.c: Add <stdarg.h>

Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Change-Id: I221a2bdb19cc7d17265c69d3fe3e1dfb490e7186
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68039
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

src/console/vtxprintf.c

@@ -6,6 +6,7 @@
 
 #include <console/vtxprintf.h>
 #include <ctype.h>
+#include <stdarg.h>
 #include <string.h>
 #include <types.h>