bpf/lib: fix void* dereference warning in lib/conntrack_test.h
bpf/lib: fix void* dereference warning in lib/conntrack_test.h

Fix the following warning:

```
gcc -I../../bpf/ -I../../bpf/include -I. -D__NR_CPUS__=2 -O2 -I../../bpf/ unit-test.c -o unit-test
In file included from unit-test.c:20:0:
../../bpf/lib/conntrack_test.h: In function ‘test___ct_lookup’:
../../bpf/lib/conntrack_test.h:88:31: warning: dereferencing ‘void *’ pointer
  struct ct_entry *entry = &map[0];
                               ^
```

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

michi-covalent/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,cilium-team/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,cilium-team/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium