os-posix: Fix build on FreeBSD

Add an include for a header required to build on recent FreeBSD.

Signed-off-by: Nathan Whitehorn <nwhitehorn@freebsd.org>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

os-posix.c

diff --git a/os-posix.c b/os-posix.c
index e3ed497..daf3d6f 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -44,6 +44,10 @@
 #include <sys/prctl.h>
 #endif
 
+#ifdef __FreeBSD__
+#include <sys/sysctl.h>
+#endif
+
 static struct passwd *user_pwd;
 static const char *chroot_dir;
 static int daemonize;