oslib-posix: Fix build on FreeBSD

Commit 10f5bff6 (util: Split out
exec_dir from os_find_datadir) moved code from os-posix.c to
util/oslib-posix.c but forgot to move a FreeBSD #include alongside,
needed for CTL_KERN among others.

Cc: Fam Zheng <famz@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Message-id: 1394717279-23406-1-git-send-email-andreas.faerber@web.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>