fix clang-20 build issue (#6103)

tools/diff/diff.cpp

diff --git a/tools/diff/diff.cpp b/tools/diff/diff.cpp
index d60edb2..349ec09 100644
--- a/tools/diff/diff.cpp
+++ b/tools/diff/diff.cpp
@@ -87,7 +87,8 @@
     return spvtools::BuildModule(
         kDefaultEnvironment, spvtools::utils::CLIMessageConsumer,
         std::string(contents.begin(), contents.end()),
-        spvtools::SpirvTools::kDefaultAssembleOption |
+        static_cast<spv_text_to_binary_options_t>(
+            spvtools::SpirvTools::kDefaultAssembleOption) |
             SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
   }