samples/main: Fix warning when TINT_BUILD_GLSL_WRITER==0

The compiler moans that the program and options parameters are not used, which can be treated as an error.

Change-Id: Idcc190204be42b722488e86a4cd354ff6a401eda
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/66445
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: Stephen White <senorblanco@chromium.org>

samples/main.cc

@@ -912,6 +912,8 @@ bool GenerateGlsl(const tint::Program* program, const Options& options) {
 
   return true;
 #else
+  (void)program;
+  (void)options;
   std::cerr << "GLSL writer not enabled in tint build" << std::endl;
   return false;
 #endif  // TINT_BUILD_GLSL_WRITER