Cirrus: Install system files in FreeBSD build jobs

Since we should check, if installing system files works correctly, we
enable the option `INSTALL_SYSTEM_FILES` in our FreeBSD build jobs
again.

.cirrus.yml

@@ -24,6 +24,7 @@ bsd_task:
     - > # We use `-std=c11`, since the header `math.h` on FreeBSD requires C11 features
       cmake
       -DBINDINGS='ALL;-io_glib'
+      -DINSTALL_SYSTEM_FILES=ON
       -DCMAKE_SKIP_INSTALL_RPATH=ON
       -DCOMMON_FLAGS=-Werror
       -DC_STD=-std=c11