android_system_memory_libion

system/memory/libion/ion.c:51:32: error: cast to smaller integer type 'ion_user_handle_t' (aka 'int') from 'void *' [-Werror,-Wvoid-pointer-to-int-cast]
        int err = ion_free(fd, (ion_user_handle_t)NULL);
                               ^~~~~~~~~~~~~~~~~~~~~~~

Bug: http://b/155835175


Test: m with aosp_blueline
Change-Id: I0d663ddbc245764b469c4f589831586ea7e55148
Signed-off-by: Simão Gomes Viana <simao@halogenos.org>