Adjust fbt_unload() definition to avoid clang 15 warning

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

    sys/cddl/dev/fbt/fbt.c:1273:11: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    fbt_unload()
              ^
               void

This is because fbt_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/fbt/fbt.c

@@ -1270,7 +1270,7 @@ fbt_load(void *dummy)
 }
 
 static int
-fbt_unload()
+fbt_unload(void)
 {
 	int error = 0;