gnu: gtkmm: Update to 3.20.0.

* gnu/packages/gtk.scm (gtkmm): Update to 3.20.0.

gnu/packages/gtk.scm

@@ -902,7 +902,7 @@ toolkit.")
 (define-public gtkmm
   (package
     (name "gtkmm")
-    (version "3.18.0")
+    (version "3.20.0")
     (source (origin
              (method url-fetch)
              (uri (string-append "mirror://gnome/sources/" name "/"
@@ -910,7 +910,7 @@ toolkit.")
                                  name "-" version ".tar.xz"))
              (sha256
               (base32
-               "0sxq700invkjpksn790gbnl8px8751kvgwn39663jx7dv89s37w2"))))
+               "12h2kd22iayvjfhmgjccm33igrbvqdj7hym31fsa1y0dhwzmf8gh"))))
     (build-system gnu-build-system)
     (native-inputs `(("pkg-config" ,pkg-config)
                      ("glib" ,glib "bin")))      ;for 'glib-compile-resources'