[Linux] Ignore deprecation of libupower call in example (#127503)

The Linux platform channels sample makes a call to the libupower
function `up_client_get_devices`, which was deprecated in libupower
0.99.8 in favour of `up_client_get_devices2`.

The prod bots have been migrated to focal, which ships libupower 0.99.11
in which the old function is deprecated and the new one is available.

The try bots have not yet been migrated and remain on bionic, which
ships libupower 0.99.7 in which the old function is not deprecated and
the new one is not available.

This marks the line in question to ignore deprecation warnings. Once all
bots have migrated to focal, we'll migrate to the new function. See
tracking issue below.

See:
https://upower.freedesktop.org/docs/UpClient.html#up-client-get-devices

Issue: https://github.com/flutter/flutter/issues/127506

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examples/platform_channel/linux/my_application.cc

@@ -217,7 +217,12 @@ static void my_application_activate(GApplication* application) {
                            G_CALLBACK(up_device_added_cb), self);
   g_signal_connect_swapped(self->up_client, "device-removed",
                            G_CALLBACK(up_device_removed_cb), self);
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+// TODO(cbracken): https://github.com/flutter/flutter/issues/127506
+// Migrate to up_client_get_devices2 once available.
   g_autoptr(GPtrArray) devices = up_client_get_devices(self->up_client);
+#pragma clang diagnostic pop
   for (guint i = 0; i < devices->len; i++) {
     g_autoptr(UpDevice) device =
         static_cast<UpDevice*>(g_ptr_array_index(devices, i));