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

@@ -497,11 +497,13 @@
  */
 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 */