staging: zcache: disable ZCACHE_DEBUG due to build error

In file included from drivers/staging/zcache/debug.c:2:0:
drivers/staging/zcache/debug.h: In function 'dec_zcache_obj_count':
drivers/staging/zcache/debug.h:16:2: error: implicit declaration of function 'BUG_ON' [-Werror=implicit-function-declaration]

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>