Suppress MSVC warning about unused variable.

One variable was only used in an 'assert' call. MSVC flagged this
as unused in Release. Suppress the warning and also add a static
cast to void so the variable becomes referenced.

BUILD.gn

@@ -152,6 +152,7 @@ source_set("glslang_sources") {
   if (is_win && !is_clang) {
     cflags = [
       "/wd4018", # signed/unsigned mismatch
+      "/wd4189", # local variable is initialized but not referenced
     ]
   }