libzpool: separate driver-side include

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17861

cmd/ztest.c

@@ -139,6 +139,7 @@
 #include <sys/crypto/icp.h>
 #include <sys/zfs_impl.h>
 #include <sys/backtrace.h>
+#include <libzpool.h>
 
 static int ztest_fd_data = -1;
 static int ztest_fd_rand = -1;