lib/utmp.c: Use the appropriate autotools macros for struct utmpx

lib/utmp.c: Use the appropriate autotools macros for struct utmpx

Recently, we started using utmpx instead of utmp, and we updated
<./configure.ac> to do the checks for 'struct utmpx' instead of
'struct utmp'.  However, I forgot to update the preprocessor
conditionals accordingly.

Fixes: 64bcb54fa962 ("lib/, src/, configure.ac: Use utmpx instead of utmp")
Link: <https://github.com/shadow-maint/shadow/pull/954>
Cc: Firas Khalil Khana <firasuke@gmail.com>
Cc: "A. Wilfox" <https://github.com/awilfox>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>