gnu: Adjust more packages to GLib "bin" split.

* gnu/packages/gimp.scm (gegl): Add glib:bin to 'native-inputs'.
* gnu/packages/mail.scm (mu): Ditto.

gnu/packages/gimp.scm

@@ -101,6 +101,7 @@ provided as well as the framework to add new color models and data types.")
        ("libjpeg" ,libjpeg-8)))
     (native-inputs
      `(("pkg-config" ,pkg-config)
+       ("glib" ,glib "bin")             ; for gtester
        ("intltool" ,intltool)))
     (home-page "http://gegl.org")
     (synopsis "Graph based image processing framework")

gnu/packages/mail.scm

@@ -301,6 +301,7 @@ repository and Maildir/IMAP as LOCAL repository.")
     (build-system gnu-build-system)
     (native-inputs
      `(("pkg-config" ,pkg-config)
+       ("glib" ,glib "bin")             ; for gtester
        ("texinfo" ,texinfo)))
     ;; TODO: Add webkit and gtk to build the mug GUI.
     (inputs