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/libshare/nfs.c

@@ -35,6 +35,7 @@
 #include <libzfs.h>
 #include <libshare.h>
 #include "libshare_impl.h"
+#include "nfs.h"
 
 static boolean_t nfs_available(void);