selftests/cgroup: cpu_hogger init: use {} instead of {NULL}

First of all, in order to build with clang at all, one must first apply
Valentin Obst's build fix for LLVM [1]. Once that is done, then when
building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...clang generates warning here, because struct cpu_hogger has multiple
fields, and the code is initializing an array of these structs, and it
is incorrect to specify a single NULL value as the initializer.

Fix this by initializing with {}, so that the compiler knows to use
default initializer values for all fields in each array entry.

[1] https://lore.kernel.org/all/20240329-selftests-libmk-llvm-rfc-v1-1-2f9ed7d1c49f@valentinobst.de/

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>