Explicitly discard released proto strings.

In newer releases of the protobuf library, the release_* methods
are marked [[nodiscard]], which causes a compilation error here.

Bug: 203713560
Test: presubmit
Change-Id: I44a9d52730bdc199e34cb25be959a955fff8f128
diff --git a/services/incremental/IncrementalService.cpp b/services/incremental/IncrementalService.cpp
index 6196c49..9c9b363 100644
--- a/services/incremental/IncrementalService.cpp
+++ b/services/incremental/IncrementalService.cpp
@@ -1287,8 +1287,8 @@
         bp.set_allocated_dest_path(&target);
         bp.set_allocated_source_subdir(&source);
         const auto metadata = bp.SerializeAsString();
-        bp.release_dest_path();
-        bp.release_source_subdir();
+        static_cast<void>(bp.release_dest_path());
+        static_cast<void>(bp.release_source_subdir());
         mdFileName = makeBindMdName();
         metadataFullPath = path::join(ifs.root, constants().mount, mdFileName);
         auto node = mIncFs->makeFile(ifs.control, metadataFullPath, 0444, idFromMetadata(metadata),