[gnome-builder] flatpak: Enable LLVM extension for use by the clang plugin

[Christian Hergert <chergert src gnome org>]

commit ee7707602acbfae436a290e0d912a7a49ab8401c
Author: vanadiae <vanadiae35 gmail com>
Date:   Fri Jul 23 21:38:40 2021 +0200

    flatpak: Enable LLVM extension for use by the clang plugin
    
    The build is broken currently because LLVM was moved to a separate
    extension in 
https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/commit/603cf4e45eb08b46a1b4a3a13c77ae8705eb5163
    So use that extension and fix the build.

 build-aux/flatpak/org.gnome.Builder.json | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index bf88ce4bc..e4965ca8e 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -3,6 +3,9 @@
     "runtime" : "org.gnome.Sdk",
     "runtime-version" : "master",
     "sdk" : "org.gnome.Sdk",
+    "sdk-extensions" : [
+        "org.freedesktop.Sdk.Extension.llvm12"
+    ],
     "command" : "gnome-builder",
     "tags" : [
         "nightly"
@@ -59,7 +62,9 @@
             "BASH_COMPLETIONSDIR" : "/app/share/bash-completion/completions",
             "MOUNT_FUSE_PATH" : "../tmp/",
             "V" : "1"
-        }
+        },
+        "prepend-path": "/usr/lib/sdk/llvm12/bin",
+        "prepend-ld-library-path": "/usr/lib/sdk/llvm12/lib"
     },
     "cleanup" : [
         "/bin/rst*",