staging: zcache/debug: compiler failure on PPC64 and revert commit.

On PPC64 we get this:
In file included from drivers/staging/zcache/debug.c:2:
drivers/staging/zcache/debug.h: In function 'dec_zcache_obj_count':
drivers/staging/zcache/debug.h:16: error: implicit declaration of function 'BUG_ON'

This simple patch adds the appropiate header file to finish
the compile and reverts
"staging: zcache: disable ZCACHE_DEBUG due to build error"
(5db5a20a)

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>