bpf: Fix warning for bpf_cpumask in verifier

Compiling with CONFIG_BPF_SYSCALL & !CONFIG_BPF_JIT throws the below
warning:

  "WARN: resolve_btfids: unresolved symbol bpf_cpumask"

Fix it by adding the appropriate #ifdef.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20240208100115.602172-1-hbathini@linux.ibm.com