Put back missing external/tarballs/ path in solenv/flatpak-manifest.in

...that accidentally got lost in 6584a5d9
"Upgrade libatomic_ops to latest libatomic_ops-7.6.8.tar.gz"

Change-Id: I79823428499efb951761c4d55eb16e154397ce60
Reviewed-on: https://gerrit.libreoffice.org/67081
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

solenv/flatpak-manifest.in

@@ -556,7 +556,7 @@
                     "url": "https://dev-www.libreoffice.org/src/libatomic_ops-7.6.8.tar.gz",
                     "sha256": "1d6a279edf81767e74d2ad2c9fce09459bc65f12c6525a40b0cb3e53c089f665",
                     "type": "file",
-                    "dest-filename": "libatomic_ops-7.6.8.tar.gz"
+                    "dest-filename": "external/tarballs/libatomic_ops-7.6.8.tar.gz"
                 },
                 {
                     "url": "https://dev-www.libreoffice.org/extern/49a64f3bcf20a7909ba2751349231d6652ded9cd2840e961b5164d09de3ffa63-opens___.ttf",