clementine: fix build by using protobuf 3.14

Upstream has fixed but has not released compatibility with 3.15 in
https://github.com/clementine-player/Clementine/pull/6927

pkgs/applications/audio/clementine/default.nix

@@ -10,7 +10,7 @@ let
   withCD = config.clementine.cd or true;
   withCloud = config.clementine.cloud or true;
 
-  # On the update after all 1.4rc, qt5.15 will be supported.
+  # On the update after all 1.4rc, qt5.15 and protobuf 3.15 will be supported.
   version = "1.4.0rc1";
 
   src = fetchFromGitHub {

pkgs/top-level/all-packages.nix

@@ -3264,6 +3264,7 @@ in
   clementine = libsForQt514.callPackage ../applications/audio/clementine {
     gst_plugins =
       with gst_all_1; [ gst-plugins-base gst-plugins-good gst-plugins-ugly gst-libav ];
+    protobuf = protobuf3_14;
   };
 
   clementineUnfree = clementine.unfree;