Enable bugprone-forward-declaration-namespace clang-tidy check

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>

.clang-tidy

@@ -1,6 +1,7 @@
 Checks: '-*,
     bugprone-argument-comment,
     bugprone-branch-clone,
+    bugprone-forward-declaration-namespace,
     cppcoreguidelines-init-variables,
     misc-*,
     -misc-non-private-member-variables-in-classes,

src/gui/systray.h

@@ -31,10 +31,6 @@ bool canOsXSendUserNotification();
 void sendOsXUserNotification(const QString &title, const QString &message);
 #endif
 
-namespace Ui {
-    class Systray;
-}
-
 /**
  * @brief The Systray class
  * @ingroup gui

test/testfolderman.cpp

@@ -17,7 +17,6 @@
 #include "testhelper.h"
 
 using namespace OCC;
-class HttpCredentials;
 
 class TestFolderMan: public QObject
 {