sudo: add fix for non-utmpx toolchains

The utmpx defines for when the target systems lacks utmpx is incomplete,
resulting in a build failure, so fix it.
This can be triggered by a uClibc toolchain that lacks UTMPX in the
configuration, or an older (<= 0.9.31) uClibc which lacks the
functionality.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>