gtk/win32: fix usage of deprecated function

gtk/win32: fix usage of deprecated function

[106/939] Compiling C object gtk/libgtk.a.p/gtkimcontextime.c.obj
../gtk/gtkimcontextime.c: In function 'gtk_im_context_ime_set_preedit_font':
../gtk/gtkimcontextime.c:780:3: warning: 'gtk_widget_get_style_context' is deprecated [-Wdeprecated-declarations]
  780 |   font_desc = gtk_css_style_get_pango_font (gtk_style_context_lookup_style (gtk_widget_get_style_context (context_ime->client_widget)));

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>