ipc: Fix unused variable error when using IPC_HANDLER_GENERIC.

If we use only IPC_HANDLER_GENERIC inside an IPC_BEGIN_MESSAGE_MAP block
we get an unused variable compile error for the extra param that's used
in IPC_BEGIN_MESSAGE_MAP_WITH_PARAM. This CL suppresses that error.

BUG=NONE

Review URL: https://codereview.chromium.org/1667353002

Cr-Commit-Position: refs/heads/master@{#373714}

ipc/ipc_message_macros.h

@@ -906,6 +906,7 @@
   { \
     typedef class_name _IpcMessageHandlerClass ALLOW_UNUSED_TYPE; \
     void* param__ = NULL; \
+    (void)param__; \
     const IPC::Message& ipc_message__ = msg; \
     switch (ipc_message__.type()) {