This fixes this warning (as appeared with commit ad8b47bff7):
[1/235] Compiling C object libgimpwidgets/libgimpwidgets-3.0.so.0.9900.17.p/gimpdialog.c.o
../../../../../../../dev/src/gimp/libgimpwidgets/gimpdialog.c:774:1: warning: ‘gimp_dialog_set_title_bar_theme’ defined but not used [-Wunused-function]
774 | gimp_dialog_set_title_bar_theme (GtkWidget *dialog)
We could either put the whole `gimp_dialog_set_title_bar_theme()` declaration,
definition and usage into #ifdef, or only the implementation (making the
function a no-op on non-Windows platforms). I chose the former. There was some
discussion that maybe some implementation may happen later for other platforms,
but until then, no need to call it needlessly (even more as we don't know when
any theoretical other implementation would happen).
