try again to silence the compiler warning about reaching the end of RestartReason.

Followup to http://https://github.com/bazelbuild/bazel/commit/1efbc1bdf3b176bf6867995fb9e4fa89735cfed0.

PiperOrigin-RevId: 253289715

src/main/cpp/blaze.cc

diff --git a/src/main/cpp/blaze.cc b/src/main/cpp/blaze.cc
index 2b128d4..3efb909 100644
--- a/src/main/cpp/blaze.cc
+++ b/src/main/cpp/blaze.cc
@@ -191,6 +191,8 @@
 
   BAZEL_DIE(blaze_exit_code::INTERNAL_ERROR)
       << "unknown RestartReason (" << reason << ").";
+  // Cannot actually reach this, but it makes the compiler happy.
+  return "unknown";
 }
 
 // Encapsulates miscellaneous information reported to the server for logging and