utils: ipc: Fix event functions with no parameters

If an event function is defined with no parameters, there would be a
compilation error complaining about unused parameters in the generated
code for the data and dataSize parameters that would normally correspond
to serialized data. Fix this by simply marking the parameters as
maybe_unused.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

utils/ipc/generators/libcamera_templates/module_ipa_proxy.cpp.tmpl

@@ -235,8 +235,8 @@ void {{proxy_name}}::recvMessage(const IPCMessage &data)
 }
 
 void {{proxy_name}}::{{method.mojom_name}}IPC(
-	std::vector<uint8_t>::const_iterator data,
-	size_t dataSize,
+	[[maybe_unused]] std::vector<uint8_t>::const_iterator data,
+	[[maybe_unused]] size_t dataSize,
 	[[maybe_unused]] const std::vector<SharedFD> &fds)
 {
 {%- for param in method.parameters %}