include <sys/vmmeter.h> to fix build
otherwise the build fails with FreeBSD 12, like
In file included from contigmem.c:57:
/usr/srcs/head/src/sys/vm/vm_phys.h:122:10: error:
use of undeclared identifier 'vm_cnt'
return (vm_cnt.v_free_count += adj);
^
Signed-off-by: Kefu Chai <tchaikov@gmail.com> Acked-by: Bruce Richardson <bruce.richardson@intel.com>