Add .lgtm.yml

Without this, lgtm.com can't successfully build the C code in Flatpak.

(cherry picked from commit 8fb7876305)

.lgtm.yml

@@ -0,0 +1,40 @@
+# vi: ts=2 sw=2 et:
+
+extraction:
+  cpp:
+    prepare:
+      # package list copied from .github/workflows/check.yml
+      packages:
+        - libglib2.0
+        - attr
+        - automake
+        - gettext
+        - autopoint
+        - bison
+        - dbus
+        - gtk-doc-tools
+        - libfuse-dev
+        - ostree
+        - libostree-dev
+        - libarchive-dev
+        - libcap-dev
+        - libattr1-dev
+        - libdw-dev
+        - libelf-dev
+        - python3-pyparsing
+        - libjson-glib-dev
+        - shared-mime-info
+        - desktop-file-utils
+        - libpolkit-agent-1-dev
+        - libpolkit-gobject-1-dev
+        - libseccomp-dev
+        - libsoup2.4-dev
+        - libsystemd-dev
+        - libxml2-utils
+        - libgpgme11-dev
+        - gobject-introspection
+        - libgirepository1.0-dev
+        - libappstream-glib-dev
+        - libdconf-dev
+        - socat
+        - libdbus-1-dev