gst_all_1.gst-plugins-good: don't allow building rpicamsrc on aarch64

MMAL is not supported on 64-bit (see [1]), so rpicamsrc cannot be built on
aarch64.

[1] raspberrypi/userland#688

pkgs/development/libraries/gstreamer/good/default.nix

@@ -45,7 +45,9 @@
 , enableDocumentation ? stdenv.hostPlatform == stdenv.buildPlatform, hotdoc
 }:
 
-assert raspiCameraSupport -> (stdenv.isLinux && (stdenv.isAarch32 || stdenv.isAarch64));
+# MMAL is not support on aarch64, see:
+# https://github.com/raspberrypi/userland/issues/688
+assert raspiCameraSupport -> (stdenv.isLinux && stdenv.isAarch32);
 
 stdenv.mkDerivation rec {
   pname = "gst-plugins-good";