virhostuptime: Add linux stub for musl

virhostuptime: Add linux stub for musl

When we want to know the boot timestamp of the host, we can call
virHostGetBootTime(). Under the hood, it uses getutxid() which is
defined by POSIX and properly check for in configure. However,
musl took a path where it declares the function but instead of
providing any useful implementation it returns NULL meaning "no
record found". If that's the case, use our second best option -
/proc/uptime and a bit of maths.

https://bugzilla.redhat.com/show_bug.cgi?id=1760885

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>