flatpak: Patch libproxy

The latest main branch contains a fix that demotes a warning to a debug
message, which solves some of our tests failing.

org.gnome.Geary.json

@@ -45,6 +45,17 @@
         "*.a"
     ],
     "modules": [
+        {
+            "name" : "libproxy",
+            "buildsystem" : "meson",
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/libproxy/libproxy.git",
+                    "branch" : "main"
+                }
+            ]
+        },
         {
             "name": "gnome-online-accounts",
             "buildsystem": "meson",