selftests: net: bpf_offload: add 'libbpf_global' to ignored maps

After installing pahole on the CI image we have a new map created
by libbpf. Ignore it otherwise we see:

  Exception: Time out waiting for map counts to stabilize want 2, have 3

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250304233204.1139251-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/bpf_offload.py

@@ -712,6 +712,7 @@ _, base_maps = bpftool("map")
 base_map_names = [
     'pid_iter.rodata', # created on each bpftool invocation
     'libbpf_det_bind', # created on each bpftool invocation
+    'libbpf_global',
 ]
 
 # Check netdevsim