Add include files for prototypes

Include the header with prototypes in the file that provides definitions
as well, to catch any mismatch between prototype and definition.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10470

module/zfs/dmu_object.c

@@ -26,6 +26,7 @@
 
 #include <sys/dbuf.h>
 #include <sys/dmu.h>
+#include <sys/dmu_impl.h>
 #include <sys/dmu_objset.h>
 #include <sys/dmu_tx.h>
 #include <sys/dnode.h>