Fix compiling with musl's utmp stubs

Fix compiling with musl's utmp stubs

This patch fixes compiling busybox with FEATURE_UTMP and _WTMP enabled.
musl, while not really support utmp/wtmp, provides stub functions, as well
as variables such as _PATH_UTMP, so that programs using utmp or wtmp can
still compile fine.

My reasoning for this patch is that on Exherbo, I'm currently trying to get
us to be able to use the same busybox config file for both glibc and musl
systems, using utmp/wtmp on systems that support it, and using the stubs
on musl without needing two different configs.

As of latest musl git, it provides all utmp functions needed; 1.1.12 doesn't,
but I sent a patch to Rich to add the utmp{,x}name functions expected to
exist, which was merged into musl upstream.

Signed-off-by: Kylie McClain <somasissounds@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>