Include C headers in introspection file.

Include C headers in introspection file.

Include gtk/gtk.h and gtk/gtk-a11y.h unconditionally,
and gtk/gtkx.h when building with X11. Ensures that
introspection data contains complete set required
headers, which is useful when generating C code based
on introspection data.

Diff for generated gir (when using X11):

```diff
   <include name="xlib" version="2.0"/>
   <package name="gtk+-3.0"/>
+  <c:include name="gtk/gtk-a11y.h"/>
+  <c:include name="gtk/gtk.h"/>
+  <c:include name="gtk/gtkx.h"/>
   <namespace name="Gtk"
              version="3.0"
```

Fixes issue #56.