selftests: af_unix: remove unused stdlib.h include

selftests: af_unix: remove unused stdlib.h include

The unix_connreset.c test included <stdlib.h>, but no symbol from that
header is used. This causes a fatal build error under certain
linux-next configurations where stdlib.h is not available.

Remove the unused include to fix the build.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202511221800.hcgCKvVa-lkp@intel.com/
Signed-off-by: Sunday Adelodun <adelodunolaoluwa@yahoo.com>
Link: https://patch.msgid.link/20251125113648.25903-1-adelodunolaoluwa@yahoo.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>