flatpak: Add libtirpc and rpcsvc-proto dependencies

`libtirpc` has been added because if in Flatpak we compile Samba
without an RPC library it will exit with the following message:
"ERROR: No rpc/rpc.h header found".

Also `rpcsvc-proto` has been added because it provides the required
`rpcgen` binary.

build-aux/flatpak/org.gnome.Settings.json

@@ -395,6 +395,28 @@
                 }
             ]
         },
+        {
+            "name" : "libtirpc",
+            "buildsystem" : "autotools",
+            "sources" : [
+                {
+                    "type" : "archive",
+                    "url" : "http://downloads.sourceforge.net/sourceforge/libtirpc/libtirpc-1.0.3.tar.bz2",
+                    "sha256" : "86c3a78fc1bddefa96111dd233124c703b22a78884203c55c3e06b3be6a0fd5e"
+                }
+            ]
+        },
+        {
+            "name" : "rpcsvc-proto",
+            "buildsystem" : "autotools",
+            "sources" : [
+                {
+                    "type" : "archive",
+                    "url" : "https://github.com/thkukuk/rpcsvc-proto/archive/v1.4.tar.gz",
+                    "sha256" : "eaa227686b84e8ae20181d2119c0ccac29d82371441844008019a134dafe3613"
+                }
+            ]
+        },
         {
             "name" : "samba",
             "buildsystem" : "autotools",