Changes to make openzfs build within FreeBSD buildworld

A collection of header changes to enable FreeBSD to build
with vendored OpenZFS.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10635

module/os/freebsd/spl/spl_vm.c

@@ -29,6 +29,8 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/types.h>
 #include <sys/param.h>
+#include <sys/counter.h>
+
 #include <sys/byteorder.h>
 #include <sys/lock.h>
 #include <sys/freebsd_rwlock.h>