cgroup-util: enforce alignment of f_handle

The change in 4d2e61211d
broke armv7 where uintptr_t is 4 bytes:

/* test_id */
Assertion '((uintptr_t) _p) % alignof(uint64_t) == 0' failed at src/basic/cgroup-util.c:108, function cg_get_cgroupid_at(). Aborting.

(gdb) p (uintptr_t)fh.file_handle.f_handle % _Alignof(uint64_t)
$9 = 4

Enforce that the structure is aligned as expected

Follow-up for 4d2e61211d

src/basic/cgroup-util.c

@@ -38,7 +38,7 @@
 typedef union {
         struct file_handle file_handle;
         uint8_t space[offsetof(struct file_handle, f_handle) + sizeof(uint64_t)];
-} cg_file_handle;
+} _alignas_(uint64_t) cg_file_handle;
 
 #define CG_FILE_HANDLE_INIT                                     \
         (cg_file_handle) {                                      \