[lldb][Python] Silence GCC warning for modules error workaround

```
lldb-python.h:16:30: warning: ‘g_fcxx_modules_workaround’ defined but not used [-Wunused-variable]
   16 | static llvm::Expected<bool> *g_fcxx_modules_workaround;
      |
```

Workaround originally added in 36cb29c.

Author: DavidSpickett

lldb/source/Plugins/ScriptInterpreter/Python/lldb-python.h

@@ -13,7 +13,7 @@
 // This declaration works around a clang module build failure.
 // It should be deleted ASAP.
 #include "llvm/Support/Error.h"
-static llvm::Expected<bool> *g_fcxx_modules_workaround;
+static llvm::Expected<bool> *g_fcxx_modules_workaround [[maybe_unused]];
 // END
 
 #include "lldb/Host/Config.h"