Add some unnecessary #include directives...

... They are not necessary to build, but the do indicate some link
dependencies to fix, that were not shown in results of scripts/graph.pl.

The link dependencies exist because the header declaring some name did not
correspond to the .cpp file that defined it.

src/commands/CommandFlag.h

@@ -120,4 +120,7 @@ struct RegisteredMenuItemEnabler{
    RegisteredMenuItemEnabler( const MenuItemEnabler &enabler );
 };
 
+// Unnecessary #include to indicate otherwise hidden link dependencies
+#include "Menus.h"
+
 #endif

src/import/ImportPlugin.h

@@ -182,4 +182,7 @@ private:
    const FileExtensions mExtensions;
 };
 
+// Unnecessary #include to indicate otherwise hidden link dependencies
+#include "Import.h"
+
 #endif

src/prefs/PrefsPanel.h

@@ -102,4 +102,7 @@ class PrefsPanel /* not final */ : public wxPanelWrapper, ComponentInterface
    virtual void Cancel();
 };
 
+// Unnecessary #include to indicate otherwise hidden link dependencies
+#include "PrefsDialog.h"
+
 #endif