tools/runqslower: Add BPF_F_CURRENT_CPU for running selftest on older kernels

Libbpf compiles and runs subset of selftests on each PR in its Github mirror
repository. To allow still building up-to-date selftests against outdated
kernel images, add back BPF_F_CURRENT_CPU definitions back.

N.B. BCC's runqslower version ([0]) doesn't need BPF_F_CURRENT_CPU due to use of
locally checked in vmlinux.h, generated against kernel with 1aae4bdd ("bpf:
Switch BPF UAPI #define constants used from BPF program side to enums")
applied.

  [0] https://github.com/iovisor/bcc/pull/2809

Fixes: 367d82f1

 (" tools/runqslower: Drop copy/pasted BPF_F_CURRENT_CPU definiton")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200311043010.530620-1-andriin@fb.com