Add proper include to silence build warning on NetBSD/FreeBSD

Reported-by: Bruce Cran <bruce@cran.org.uk>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/os/os-freebsd.h b/os/os-freebsd.h
index fad051f..317d403 100644
--- a/os/os-freebsd.h
+++ b/os/os-freebsd.h
@@ -4,6 +4,7 @@
 #include <errno.h>
 #include <sys/sysctl.h>
 #include <sys/disk.h>
+#include <sys/thr.h>
 
 #include "../file.h"