ANDROID: mm/memory.c: export mm_trace_rss_stat

to workaround following error:
    ERROR: "mm_trace_rss_stat" [drivers/gpu/arm_gpu/mali_kbase.ko] undefined!
reported when make modules

Test: test 5.5.0-rc1-mainline with hikey960 with GKI method

Change-Id: Icb56b0c760780c3f9081f82d921406d17f4f1c08
[adelva: workaround for Mali driver on hikey960]
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Signed-off-by: Alistair Delva <adelva@google.com>