include: Move snprintf to stdio.h

Move snprintf to stdio.h since it is needed by exteranl libraries.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

lib/fwu_updates/fwu_mtd.c

@@ -11,7 +11,7 @@
 #include <malloc.h>
 #include <mtd.h>
 #include <uuid.h>
-#include <vsprintf.h>
+#include <stdio.h>
 
 #include <dm/ofnode.h>