gnu: waybar: Fix build.

Reported by bdju.

* gnu/packages/pretty-print.scm (fmt-6): New variable.
* gnu/packages/wm.scm (waybar)[inputs]: Replace fmt with fmt-6.

gnu/packages/pretty-print.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2016, 2019 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2016, 2019, 2020 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2016 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2017 Marius Bakke <mbakke@fastmail.com>
 ;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
@@ -190,6 +190,19 @@ to @code{IOStreams}.")
     ;; The library is bsd-2, but documentation and tests include other licenses.
     (license (list bsd-2 bsd-3 psfl))))
 
+(define-public fmt-6
+  (package
+    (inherit fmt)
+    (name "fmt")
+    (version "6.2.1")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (string-append "https://github.com/fmtlib/fmt/releases/download/"
+                           version "/fmt-" version ".zip"))
+       (sha256
+        (base32 "06l8g59frbsbwj15kg6x2bbn6p8yidh6wzsigdhbdjncvm1agzll"))))))
+
 (define-public source-highlight
   (package
     (name "source-highlight")

gnu/packages/wm.scm

@@ -5,7 +5,7 @@
 ;;; Copyright © 2015 xd1le <elisp.vim@gmail.com>
 ;;; Copyright © 2015 Paul van der Walt <paul@denknerd.org>
 ;;; Copyright © 2016 Danny Milosavljevic <dannym@scratchpost.org>
-;;; Copyright © 2016, 2019 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2016, 2019, 2020 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2016 Al McElrath <hello@yrns.org>
 ;;; Copyright © 2016 Carlo Zancanaro <carlo@zancanaro.id.au>
 ;;; Copyright © 2016, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
@@ -1526,7 +1526,7 @@ modules for building a Wayland compositor.")
         (base32 "0ks719khhg2zwpyiwa2079i6962qcxpapm28hmr4ckpsp2n659ck"))))
     (build-system meson-build-system)
     (inputs `(("date" ,date)
-              ("fmt" ,fmt)
+              ("fmt" ,fmt-6)
               ("gtk-layer-shell" ,gtk-layer-shell)
               ("gtkmm" ,gtkmm)
               ("jsoncpp" ,jsoncpp)