Explicitly ignore the result of std::async

The newer libc++ marks std::async with [[nodiscard]] in C++20 mode.

Bug: b/175635923
Test: treehugger
Change-Id: Ib63259983d952b97cf1b1c6c243c831cb72f9724
diff --git a/fs_mgr/libsnapshot/snapuserd/dm-snapshot-merge/snapuserd.cpp b/fs_mgr/libsnapshot/snapuserd/dm-snapshot-merge/snapuserd.cpp
index 978a7f2..71664bf 100644
--- a/fs_mgr/libsnapshot/snapuserd/dm-snapshot-merge/snapuserd.cpp
+++ b/fs_mgr/libsnapshot/snapuserd/dm-snapshot-merge/snapuserd.cpp
@@ -734,8 +734,8 @@
     off_t offset = 0;
 
     for (int i = 0; i < num_threads; i++) {
-        std::async(std::launch::async, &Snapuserd::ReadBlocksToCache, this, dm_block_device,
-                   partition_name, offset, read_sz_per_thread);
+        (void)std::async(std::launch::async, &Snapuserd::ReadBlocksToCache, this, dm_block_device,
+                         partition_name, offset, read_sz_per_thread);
 
         offset += read_sz_per_thread;
     }