Adjust prototype_unload() definition to avoid clang 15 warning

With clang 15, the following -Werror warnings is produced:

    sys/cddl/dev/prototype.c:99:17: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    prototype_unload()
                    ^
                     void

This is because prototype_unload() is declared with a (void) argument
list, but defined with an empty argument list. Make the definition match
the declaration.

MFC after:	3 days

sys/cddl/dev/prototype.c

@@ -96,7 +96,7 @@ prototype_load(void *dummy)
 
 
 static int
-prototype_unload()
+prototype_unload(void)
 {
 	int error = 0;