libcgroup: Use the correct data type in the get_all_controller test case

libcgroup: Use the correct data type in the get_all_controller test case

get_all_controller.c: In function ‘main’:
get_all_controller.c:18: warning: passing argument 1 of ‘cgroup_get_all_controller_begin’ from incompatible pointer type
get_all_controller.c:23: warning: passing argument 1 of ‘cgroup_get_all_controller_next’ from incompatible pointer type
get_all_controller.c:31: warning: passing argument 1 of ‘cgroup_get_all_controller_end’ from incompatible pointer type

The test case used a FILE * instead of a void *. Correct this change (The datatype is opaque to the caller)

Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>