Fix signature for private functions without header declarations

Clang will complain if a function has no prior declaration

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9467

module/zfs/btree.c

@@ -59,7 +59,7 @@ int zfs_btree_verify_intensity = 0;
  * A convenience function to silence warnings from memmove's return value and
  * change argument order to src, dest.
  */
-void
+static void
 bmov(const void *src, void *dest, size_t size)
 {
 	(void) memmove(dest, src, size);

module/zfs/dsl_destroy.c

@@ -938,7 +938,7 @@ dsl_async_clone_destroy(dsl_dataset_t *ds, dmu_tx_t *tx)
  * Move the bptree into the pool's list of trees to clean up, update space
  * accounting information and destroy the zil.
  */
-void
+static void
 dsl_async_dataset_destroy(dsl_dataset_t *ds, dmu_tx_t *tx)
 {
 	uint64_t used, comp, uncomp;