cgroup-util: move Set* allocation into cg_kernel_controllers()

Previously, callers had to do this on their own. Let's make the call do
that instead, making the caller code a bit shorter.

src/basic/cgroup-util.h

@@ -243,7 +243,7 @@ int cg_mask_supported(CGroupMask *ret);
 int cg_mask_from_string(const char *s, CGroupMask *ret);
 int cg_mask_to_string(CGroupMask mask, char **ret);
 
-int cg_kernel_controllers(Set *controllers);
+int cg_kernel_controllers(Set **controllers);
 
 bool cg_ns_supported(void);