console: Expose vsnprintf

It's a standard function.

Change-Id: I039cce2dfc4e168804eb7d12b76a29af712ac7a1
Signed-off-by: David Hendricks <dhendricks@fb.com>
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/23616
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/console/vsprintf.c

@@ -34,7 +34,7 @@ static void str_tx_byte(unsigned char byte, void *data)
 	}
 }
 
-static int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
+int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 {
 	int i;
 	struct vsnprintf_context ctx;