gnu: qtbase: Enable GTK support.

This allows Qt applications to use the GTK themes present on the system,
providing a more integrated look and feel for many Qt applications used in the
context of a GTK/GNOME environment.

* gnu/packages/qt.scm (qtbase-5)[inputs]: Add gtk+.

gnu/packages/qt.scm

@@ -338,6 +338,7 @@ system, and the core design of Django is reused in Grantlee.")
        ("fontconfig" ,fontconfig)
        ("freetype" ,freetype)
        ("glib" ,glib)
+       ("gtk+" ,gtk+)                   ;for GTK theme support
        ("harfbuzz" ,harfbuzz)
        ("icu4c" ,icu4c)
        ("libinput" ,libinput-minimal)