Building LibreOffice needs gperf 3.1 now

...since e8e3b00b "Require at least gperf 3.1,
which no longer emits 'register'", but org.freedesktop.Sdk//1.6 only has "GNU
gperf 3.0.4" (and org.freedesktop.Sdk//18.08 also only has that version, so this
fix will remain relevant even when
<https://github.com/flathub/org.libreoffice.LibreOffice/issues/54> "Build
against org.freedesktop.Platform 18.08" eventually gets addressed).

Change-Id: I5a4fc6692aa077c989348903a3ed1f178922cfcb

solenv/flatpak-manifest.in

@@ -56,6 +56,17 @@
                 }
             ]
         },
+        {
+            "name": "gperf",
+            "cleanup": [ "*" ],
+            "sources": [
+                {
+                    "type": "archive",
+                    "url": "http://ftp.gnu.org/pub/gnu/gperf/gperf-3.1.tar.gz",
+                    "sha256": "588546b945bba4b70b6a3a616e80b4ab466e3f33024a352fc2198112cdbb3ae2"
+                }
+            ]
+        },
         {
             "name": "libreoffice",
             "sources": [