vm/vm_page.h: Fix trivial '-Wpointer-sign' warning

pq_vcnt, as a count of real things, has no business being negative. It is only
ever initialized by a u_int counter.

The warning came from the atomic_add_int() in vm_pagequeue_cnt_add().

Rectify the warning by changing the variable to u_int. No functional change.

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