sys/vmmeter.h: Fix trivial '-Wsign-compare' warning in common header

Frankly, it doesn't make sense for vm_pageout_wakeup_thresh to have a negative
value (it is only ever set to a fraction of v_free_min, which is unsigned and
also obviously non-negative). But I'm not going to try and convert every
non-negative scalar in the VM to unsigned today, so just cast it for the
comparison.

Submitted by: Clang 3.3
Sponsored by: EMC / Isilon Storage Division