staging: fix zcache building

zcache is only building tmem.c and not building zcache.c. To keep the
module name, zcache.c must be renamed if symbols from tmem.c are to
remain unexported.
Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Acked-by: NDan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/zcache/Makefile

-zcache-y	:=	tmem.o
+zcache-y	:=	zcache-main.o tmem.o
 
 obj-$(CONFIG_ZCACHE)	+=	zcache.o