gnu: libnotify: Propagate required inputs.

libnotify doesn't propagate other needed libraries for linking against
it resulting in pkg-config errors like

> Package gdk-pixbuf-2.0 was not found in the pkg-config search path.
> Perhaps you should add the directory containing `gdk-pixbuf-2.0.pc'
> to the PKG_CONFIG_PATH environment variable

* gnu/packages/gnome.scm (libnotify)[inputs]: Move GLIB and GDK-PIXBUF ...
[propagated-inputs]: ... here.  New field.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>