virInitctlRequest: Don't hardcode 384 bytes size

When MAXHOSTNAMELEN is set we have to take it's value into account.
Otherwise the build fails on kFreeBSD (FreeBSD kernel and GNU userland)

src/util/virinitctl.c

@@ -103,7 +103,11 @@ struct virInitctlRequest {
     } i;
 };
 
-verify(sizeof(struct virInitctlRequest) == 384);
+# ifdef MAXHOSTNAMELEN
+  verify(sizeof(struct virInitctlRequest) == 320 + MAXHOSTNAMELEN);
+# else
+  verify(sizeof(struct virInitctlRequest) == 384);
+#endif
 
 /*
  * Send a message to init to change the runlevel