ath6kl: include vmalloc.h in debug.c

Fixes compilation errors when compiling for ARM:

ath6kl/debug.c:312: error: implicit declaration of function 'vmalloc'
ath6kl/debug.c:312: warning: assignment makes pointer from integer without a cast
ath6kl/debug.c:342: error: implicit declaration of function 'vfree'
ath6kl/debug.c:696: warning: assignment makes pointer from integer without a cast
ath6kl/debug.c:871: warning: assignment makes pointer from integer without a cast

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>