entry.hg: Mark GTKMM_API for the Entry class

Hi,

This attempts to remove the warnings that were raised since the API deprecations, as now the GtkEntry class should be marked with GTKMM_API instead.

c:\gnome.build.unstable\gtkmm4.b\gtk\gtkmm/searchentry.h(76): warning C4275: non dll-interface class 'Gtk::Entry' used as base for dll-interface class 'Gtk::SearchEntry'
c:\gnome.build.unstable\gtkmm4.b\gtk\gtkmm/entry.h(66): note: see declaration of 'Gtk::Entry'
c:\gnome.build.unstable\gtkmm4.b\gtk\gtkmm/searchentry.h(74): note: see declaration of 'Gtk::SearchEntry'

With blessings, thank you!