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>
diff --git a/common/button_cmd.c b/common/button_cmd.c
index 8642c26..72dac1f 100644
--- a/common/button_cmd.c
+++ b/common/button_cmd.c
@@ -8,7 +8,7 @@
 #include <command.h>
 #include <env.h>
 #include <log.h>
-#include <vsprintf.h>
+#include <stdio.h>
 
 /* Some sane limit "just in case" */
 #define MAX_BTN_CMDS 32