os/grow: define 'p' under the same ifdef as it's consumed

*** 497,507 ****
--- 497,509 ----
   */
  int
  choose_addr(struct as *as, caddr_t *addrp, size_t len, offset_t off,
      int vacalign, uint_t flags)
  {
+ #if defined(__amd64)
          proc_t *p = curproc;
+ #endif
          caddr_t basep = (caddr_t)(uintptr_t)((uintptr_t)*addrp & PAGEMASK);
          size_t lenp;
  
          ASSERT(AS_ISCLAIMGAP(as));      /* searches should be serialized */