ui-previewer needs the tk library, too

Prevent ui-previewer from crashing. I have no idea why this is needed and how
it works. Or why there is no useful or even useless warning or error message
at all.

Change-Id: I461a611a6646b1d6c524fc04624e6d780eb7282a

vcl/Executable_ui-previewer.mk

@@ -25,6 +25,7 @@ $(eval $(call gb_Executable_use_libraries,ui-previewer,\
 	cppu \
 	cppuhelper \
 	sal \
+	tk \
     tl \
     ucbhelper \
     vcl \