drivers: staging: zcache: fix compile error

Fix below compile error:
drivers/built-in.o: In function `zcache_pampd_free':
>> zcache-main.c:(.text+0xb1c8a): undefined reference to `ramster_pampd_free'
>> zcache-main.c:(.text+0xb1cbc): undefined reference to `ramster_count_foreign_pages'
drivers/built-in.o: In function `zcache_pampd_get_data_and_free':
>> zcache-main.c:(.text+0xb1f05): undefined reference to `ramster_count_foreign_pages'
drivers/built-in.o: In function `zcache_cpu_notifier':
>> zcache-main.c:(.text+0xb228d): undefined reference to `ramster_cpu_up'
>> zcache-main.c:(.text+0xb2339): undefined reference to `ramster_cpu_down'
drivers/built-in.o: In function `zcache_pampd_create':
>> (.text+0xb26ce): undefined reference to `ramster_count_foreign_pages'
drivers/built-in.o: In function `zcache_pampd_create':
>> (.text+0xb27ef): undefined reference to `ramster_count_foreign_pages'
drivers/built-in.o: In function `zcache_put_page':
>> (.text+0xb299f): undefined reference to `ramster_do_preload_flnode'
drivers/built-in.o: In function `zcache_flush_page':
>> (.text+0xb2ea3): undefined reference to `ramster_do_preload_flnode'
drivers/built-in.o: In function `zcache_flush_object':
>> (.text+0xb307c): undefined reference to `ramster_do_preload_flnode'
drivers/built-in.o: In function `zcache_init':
>> zcache-main.c:(.text+0xb3629): undefined reference to `ramster_register_pamops'
>> zcache-main.c:(.text+0xb3868): undefined reference to `ramster_init'
>> drivers/built-in.o:(.rodata+0x15058): undefined reference to `ramster_foreign_eph_pages'
>> drivers/built-in.o:(.rodata+0x15078): undefined reference to `ramster_foreign_pers_pages'

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>