map.lookup() does not accept a pointer to const

A bit confusing.

Code:

```C
BPF_HASH(children_map, u64, u64);
...
{
u64 pid_tid = bpf_get_current_pid_tgid();
const u64 pid = (pid_tid >> 32);

u64 *val = children_map.lookup(&pid);
}
```
Message:

```
main.c:28515:34: warning: passing 'const u64 *' (aka 'const unsigned long
long *') to parameter of type 'u64 *' (aka 'unsigned long long *') discards
qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
u64 *val = children_map.lookup(&pid);
^~~~
```

Removing const avoid warning message

Reproduce the compiler warning with the BPF_HASH example, then trace the map.lookup() declaration and implementation used by BCC. Done means a pointer to const u64 is accepted without the discarded-qualifiers warning while preserving the existing lookup behavior.

Written by the indexing model from the issue text.