LSP: Fix potential build error with clang

Builds fine with GCC, but Clang Code Model complains:

  languagefeatures.h:437:73: error: use of undeclared identifier
    'nullptr_t'

Adapt to other nullptr_t uses in this file and prefix with "std::".

Change-Id: Icc4b8e045a9bfb94d4e7535bf884641116ae7f7d
Reviewed-by: David Schulz <david.schulz@qt.io>

src/libs/languageserverprotocol/languagefeatures.h

@@ -434,7 +434,7 @@ public:
 };
 
 class LANGUAGESERVERPROTOCOL_EXPORT CodeActionResult
-    : public Utils::variant<QList<Utils::variant<Command, CodeAction>>, nullptr_t>
+    : public Utils::variant<QList<Utils::variant<Command, CodeAction>>, std::nullptr_t>
 {
 public:
     using variant::variant;