tty: Avoid the use of one-element arrays

One-element arrays are being deprecated[1]. Replace the one-element arrays
with simple value types 'char reserved_char' and 'compat_int_t reserved'[2],
once it seems these are just placeholders for alignment.

[1] KSPP#79
[2] KSPP#86

Tested-by: kernel test robot <lkp@intel.com>
Link: https://github.com/GustavoARSilva/linux-hardening/blob/master/cii/0-day/tty-20200716.md
Acked-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/tty/tty_io.c

@@ -2683,15 +2683,15 @@ struct serial_struct32 {
         compat_int_t    baud_base;
         unsigned short  close_delay;
         char    io_type;
-        char    reserved_char[1];
+        char    reserved_char;
         compat_int_t    hub6;
         unsigned short  closing_wait; /* time to wait before closing */
         unsigned short  closing_wait2; /* no longer used... */
         compat_uint_t   iomem_base;
         unsigned short  iomem_reg_shift;
         unsigned int    port_high;
      /* compat_ulong_t  iomap_base FIXME */
-        compat_int_t    reserved[1];
+        compat_int_t    reserved;
 };
 
 static int compat_tty_tiocsserial(struct tty_struct *tty,