fix(flatpak): add llvm18 SDK extension for bindgen/libclang

pipewire-rs uses bindgen which requires libclang at build time.
Add org.freedesktop.Sdk.Extension.llvm18 to sdk-extensions and set
LIBCLANG_PATH=/usr/lib/sdk/llvm18/lib in build-options. Pre-install
the extension in CI alongside the Rust extension.

Author: thewrz

.github/workflows/flatpak.yml

@@ -28,7 +28,8 @@ jobs:
           flatpak remote-add --if-not-exists flathub \
             https://flathub.org/repo/flathub.flatpakrepo
           flatpak install --noninteractive flathub \
-            org.freedesktop.Sdk.Extension.rust-stable//24.08
+            org.freedesktop.Sdk.Extension.rust-stable//24.08 \
+            org.freedesktop.Sdk.Extension.llvm18//24.08
 
       - name: Build with flatpak-builder
         run: |

packaging/flatpak/io.github.thewrz.HonkHonk.yml

@@ -4,12 +4,14 @@ runtime-version: '24.08'
 sdk: org.freedesktop.Sdk
 sdk-extensions:
   - org.freedesktop.Sdk.Extension.rust-stable
+  - org.freedesktop.Sdk.Extension.llvm18
 command: honkhonk
 
 build-options:
-  append-path: /usr/lib/sdk/rust-stable/bin
+  append-path: /usr/lib/sdk/rust-stable/bin:/usr/lib/sdk/llvm18/bin
   env:
     CARGO_HOME: /run/build/honkhonk/cargo
+    LIBCLANG_PATH: /usr/lib/sdk/llvm18/lib
 
 finish-args:
   # Wayland display