Explicitly discard released proto strings in test.

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: If8c6392a5d36014f1839deaaf241b636610add71
diff --git a/services/incremental/test/IncrementalServiceTest.cpp b/services/incremental/test/IncrementalServiceTest.cpp
index 59d96d2..d9d3d62 100644
--- a/services/incremental/test/IncrementalServiceTest.cpp
+++ b/services/incremental/test/IncrementalServiceTest.cpp
@@ -474,8 +474,8 @@
         m.mutable_loader()->set_package_name("com.test");
         m.mutable_loader()->set_arguments("com.uri");
         const auto metadata = m.SerializeAsString();
-        m.mutable_loader()->release_arguments();
-        m.mutable_loader()->release_package_name();
+        static_cast<void>(m.mutable_loader()->release_arguments());
+        static_cast<void>(m.mutable_loader()->release_package_name());
         return {metadata.begin(), metadata.end()};
     }
     RawMetadata getStorageMetadata(const Control& control, std::string_view path) {
@@ -492,8 +492,8 @@
         bp.set_allocated_dest_path(&destPath);
         bp.set_allocated_source_subdir(&srcPath);
         const auto metadata = bp.SerializeAsString();
-        bp.release_source_subdir();
-        bp.release_dest_path();
+        static_cast<void>(bp.release_source_subdir());
+        static_cast<void>(bp.release_dest_path());
         return std::vector<char>(metadata.begin(), metadata.end());
     }
 };