[SPIR-V] Fix build with -DENABLE_SPIRV_CODEGEN=OFF (#6933)

A 'using' was not gated.

Signed-off-by: Nathan Gauër <brioche@google.com>

lib/DxcSupport/HLSLOptions.cpp

@@ -31,7 +31,9 @@ using namespace llvm::opt;
 using namespace dxc;
 using namespace hlsl;
 using namespace hlsl::options;
+#ifdef ENABLE_SPIRV_CODEGEN
 using namespace clang::spirv;
+#endif
 
 #define PREFIX(NAME, VALUE) static const char *const NAME[] = VALUE;
 #include "dxc/Support/HLSLOptions.inc"