Multisink.c: Clear -Werror=incompatible-pointer-types error

MultiSink.c: In function ‘SetValues’:
MultiSink.c:582:18: error: passing argument 1 of ‘SetTabs’ from
 incompatible pointer type [-Werror=incompatible-pointer-types]
  582 |         SetTabs( w, w->text_sink.tab_count, w->text_sink.char_tabs );
      |                  ^
      |                  |
      |                  MultiSinkObject {aka struct _MultiSinkRec *}
MultiSink.c:99:21: note: expected ‘Widget’ {aka ‘struct _WidgetRec *’}
 but argument is of type ‘MultiSinkObject’ {aka ‘struct _MultiSinkRec *’}
   99 | static void SetTabs(Widget, int, short *);
      |                     ^~~~~~

Adopts change originally made to libXaw in XFree86 4