selftest: af_unix: Fix kselftest compilation warnings

selftest: af_unix: Fix kselftest compilation warnings

Change expected_buf from (const void *) to (const char *)
in function __recvpair().
This change fixes the below warnings during test compilation:

```
In file included from msg_oob.c:14:
msg_oob.c: In function ‘__recvpair’:

../../kselftest_harness.h:106:40: warning: format ‘%s’ expects argument
of type ‘char *’,but argument 6 has type ‘const void *’ [-Wformat=]

../../kselftest_harness.h:101:17: note: in expansion of macro ‘__TH_LOG’
msg_oob.c:235:17: note: in expansion of macro ‘TH_LOG’

../../kselftest_harness.h:106:40: warning: format ‘%s’ expects argument
of type ‘char *’,but argument 6 has type ‘const void *’ [-Wformat=]

../../kselftest_harness.h:101:17: note: in expansion of macro ‘__TH_LOG’
msg_oob.c:259:25: note: in expansion of macro ‘TH_LOG’
```

Fixes: d098d77232c3 ("selftest: af_unix: Add msg_oob.c.")
Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240814080743.1156166-1-jain.abhinav177@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>