ath5k: Add missing vmalloc.h include.

After merging the wireless-next tree, today's linux-next build (powerpc
allyesconfig) failed like this:

drivers/net/wireless/ath/ath5k/debug.c: In function 'open_file_eeprom':
drivers/net/wireless/ath/ath5k/debug.c:933:2: error: implicit declaration of function 'vmalloc' [-Werror=implicit-function-declaration]
  buf = vmalloc(eesize);
  ^
drivers/net/wireless/ath/ath5k/debug.c:933:6: warning: assignment makes pointer from integer without a cast
  buf = vmalloc(eesize);
      ^
drivers/net/wireless/ath/ath5k/debug.c:960:2: error: implicit declaration of function 'vfree' [-Werror=implicit-function-declaration]
  vfree(buf);
  ^

Caused by commit db906eb2 ("ath5k: added debugfs file for dumping
eeprom").  Also reported by Guenter Roeck.

I have used Geert Uytterhoeven's suggested fix of including vmalloc.h
and so added this patch for today:

From: Stephen Rothwell <sfr@canb.auug.org.au>
Date: Mon, 8 Sep 2014 18:39:23 +1000
Subject: [PATCH] ath5k: fix debugfs addition

Reported-by: Guenter Roeck <linux@roeck-us.net>
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>