ci: install systemd-dev for systemd.pc in build-full job

On Debian sid, `dependency('systemd')` needs systemd.pc, which is shipped by
the `systemd-dev` package, not `libsystemd-dev`. Add it so the forced
-Dsystemd=enabled configure step succeeds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/linux.yml

@@ -58,6 +58,7 @@ jobs:
             libmm-glib-dev \
             libevdev-dev \
             libsystemd-dev \
+            systemd-dev \
             libepoxy-dev \
             libfftw3-dev
       - name: configure