flatpak: Temporarily bundle libunwind

Some of our `ComposerWidgetTest` tests are failing with SIGSEGV.

This seemingly happens because they create a new
`Geary.AccountProblemReport`; for example, because a `MockAccount`
doesn't actually implement `get_required_special_folder_async()`.

That in turns will indirectly create a `Geary.ErrorContext` which will
try to use libunwind to create a stack trace. And there, it seems that
`unw_get_proc_name()` is the function that is actually segfaulting.

Looking at libunwind recent history, a similar crash was reported [1]
which has been fixed in libunwind commit b32ce6ee36. Since that hasn't
been backported into a release, let's bundle the latest mainline from
libunwind instead to get the fixes in.

[1]: https://github.com/libunwind/libunwind/issues/827

org.gnome.Geary.json

@@ -45,6 +45,22 @@
         "*.a"
     ],
     "modules": [
+        {
+            "name": "libunwind",
+            "sources": [
+                {
+                    "type": "git",
+                    "url": "https://github.com/libunwind/libunwind.git",
+                    "branch": "master"
+                },
+                {
+                    "type": "script",
+                    "commands": [
+                        "autoreconf -sif"
+                    ]
+                }
+            ]
+        },
         {
             "name" : "libproxy",
             "buildsystem" : "meson",