d/control: Add ${gir:Depends}, ${gir:Provides} substvars

With a newer version of gobject-introspection (currently in experimental),
these will supersede some of the current hard-coded Depends and Provides.

debian/control

@@ -169,6 +169,7 @@ Depends: gir1.2-gtk-3.0 (= ${binary:Version}),
          libxrandr-dev,
          pkg-config,
          wayland-protocols (>= 1.17) [linux-any],
+         ${gir:Depends},
          ${misc:Depends},
          ${shlibs:Depends},
          ${cloudProvider:Depends},
@@ -176,6 +177,7 @@ Depends: gir1.2-gtk-3.0 (= ${binary:Version}),
          libgtk-3-0 (= ${binary:Version})
 Provides: gir1.2-gdk-3.0-dev (= ${binary:Version}),
           gir1.2-gdkx11-3.0-dev (= ${binary:Version}),
+          ${gir:Provides}
 Suggests: libgtk-3-doc
 Description: development files for the GTK library
  GTK is a multi-platform toolkit for creating graphical user
@@ -231,7 +233,8 @@ Depends: ${gir:Depends},
 Breaks: python-gi (<< 3.18),
         python3-gi (<< 3.18)
 Provides: gir1.2-gdk-3.0 (= ${binary:Version}),
-          gir1.2-gdkx11-3.0 (= ${binary:Version})
+          gir1.2-gdkx11-3.0 (= ${binary:Version}),
+          ${gir:Provides}
 Description: GTK graphical user interface library -- gir bindings
  GTK is a multi-platform toolkit for creating graphical user
  interfaces. Offering a complete set of widgets, GTK is suitable