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

lib/libzfs/os/freebsd/libzfs_zmount.c

@@ -43,6 +43,8 @@ __FBSDID("$FreeBSD$");
 #include <sys/errno.h>
 #include <libzfs.h>
 
+#include "libzfs_impl.h"
+
 static void
 build_iovec(struct iovec **iov, int *iovlen, const char *name, void *val,
     size_t len)