Add acksync helper to wait for acks

This will allow to send requests with sequence numbers to the server
and wait for acknowledgements.

PR #2814 <https://github.com/Genymobile/scrcpy/pull/2814>

app/meson.build

@@ -25,6 +25,7 @@ src = [
     'src/server.c',
     'src/stream.c',
     'src/video_buffer.c',
+    'src/util/acksync.c',
     'src/util/file.c',
     'src/util/intr.c',
     'src/util/log.c',