printf: fix %% doubling down as a format specifier

When parsing a %% format, the second % character was mistakenly not
skipped over after emitting a '%' output; this resulted in it being
treated as a format specifier. For instance,

  printf("%%d", 12);

would print "%12".

src/std/stdio.c

@@ -227,6 +227,7 @@ void kprint(char const *format, va_list *args)
 		if(c == '%')
 		{
 			kprint_out('%');
+			format++;
 			continue;
 		}