qemu-ga: check if utmpx.h is available on the system

Commit 161a56a9 added command guest-get-users and requires the
utmpx.h (defined by POSIX) to work. It is however not always available
(e.g. on OpenBSD) therefor a check for its existence is necessary.

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>