Fix a Wvoid-pointer-to-enum-cast warning missed in r359978.

This pattern is used in callbacks with void * data arguments and seems
both relatively uncommon and relatively harmless.  Silence the warning
by casting through uintptr_t.

This warning is on by default in Clang 11.

MFC after:	3 days
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D24425

cddl/contrib/opensolaris/tools/ctf/cvt/iidesc.c

@@ -179,7 +179,7 @@ int
 iidesc_count_type(void *data, void *private)
 {
 	iidesc_t *ii = data;
-	iitype_t match = (iitype_t)private;
+	iitype_t match = (iitype_t)(uintptr_t)private;
 
 	return (ii->ii_type == match);
 }