Fix robtk.mk dependency on PLUGIN_SOURCE

[domohawk]

The UI targets in robtk.mk should include a dependency on PLUGIN_SOURCE, so that if it is modified, the makefile will rebuild.

I've tried adding variants such as gui/$(*F).c, $(value $(*F)_UISRC) and gui/%.c to L27 but I can't get the makefile fu right. hardcoding the filename (e.g. gui/rtkfil.c) works however.