Mallets: build properly with Stk 4.4

As of Stk 4.4 all classes and types have been moved to the namespace "stk".
This breaks build of the Mallets plugin. This can be easily fixed by
using the namespace "stk". For compatibility reasons we have to declare
the namespace before, because it did not exist in previous versions.

Closes #2870878.

plugins/stk/mallets/mallets.h

@@ -36,6 +36,12 @@
 #include "note_play_handle.h"
 #include "led_checkbox.h"
 
+// As of Stk 4.4 all classes and types have been moved to the namespace "stk".
+// However in older versions this namespace does not exist, therefore declare it
+// so this plugin builds with all versions of Stk.
+namespace stk { } ;
+using namespace stk;
+
 
 class malletsSynth
 {