vsprintf.c: Always enable CONFIG_SYS_VSNPRINTF

Enabling this function always removes some class of string saftey issues.
The size change here in general is about 400 bytes and this seems a reasonable
trade-off.

Cc: Peng Fan <peng.fan@nxp.com>
Cc: Peter Robinson <pbrobinson@gmail.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Adrian Alonso <aalonso@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tom Rini <trini@konsulko.com>