bpf: selftests: Fix bpf_cpumask_first_zero() kfunc prototype

bpf: selftests: Fix bpf_cpumask_first_zero() kfunc prototype

The prototype in progs/nested_trust_common.h is not in line with how the
actual kfuncs are defined in kernel/bpf/cpumask.c. This causes compilation
errors when kfunc prototypes are generated from BTF.

Fix by aligning with actual kfunc definitions.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/437936a4e554b02e04566dd6e3f0a5d08370cc8c.1718207789.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <ast@kernel.org>