Add a final return to RestartReason.

This silences the warning:
INFO: From Compiling src/main/cpp/blaze.cc:
src/main/cpp/blaze.cc: In function 'const char* blaze::ReasonString(blaze::RestartReason)':
src/main/cpp/blaze.cc:191:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^
PiperOrigin-RevId: 252892834

src/main/cpp/blaze.cc

diff --git a/src/main/cpp/blaze.cc b/src/main/cpp/blaze.cc
index 2208970..e4cd2ed 100644
--- a/src/main/cpp/blaze.cc
+++ b/src/main/cpp/blaze.cc
@@ -188,6 +188,9 @@
     case SERVER_UNRESPONSIVE:
       return "server_unresponsive";
   }
+
+  BAZEL_DIE(blaze_exit_code::INTERNAL_ERROR)
+      << "unknown RestartReason (" << reason << ").";
 }
 
 // Encapsulates miscellaneous information reported to the server for logging and