build: Fix *.directory translations

gettext does not know this file format:

/usr/bin/xgettext: warning: file 'data/xfce4-screensaver.directory.in' extension 'directory' is unknown; will try C

Tell them this can be treated as desktop file by hacking around the file suffix.

This is the same thing done in mate-screensaver: https://github.com/mate-desktop/mate-screensaver/commit/bce4817d3c2ddc150582db8451f3d92a5a2df5da

Here is the diff for the generated POT file:

--- xfce4-screensaver.pot0      2024-04-07 22:47:08.291729426 +0800
+++ xfce4-screensaver.pot1      2024-04-07 22:45:42.252643812 +0800
@@ -6,9 +6,9 @@
 #, fuzzy
 msgid ""
 msgstr ""
-"Project-Id-Version: xfce4-screensaver 4.18.3git-bae2596\n"
+"Project-Id-Version: xfce4-screensaver 4.18.3git-49fc639\n"
 "Report-Msgid-Bugs-To: https://gitlab.xfce.org/apps/xfce4-screensaver\n"
-"POT-Creation-Date: 2024-04-07 22:47+0800\n"
+"POT-Creation-Date: 2024-04-07 22:45+0800\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -32,6 +32,14 @@
 msgid "Screensaver"
 msgstr ""
 
+#: data/xfce4-screensaver.directory.desktop.in:4
+msgid "Screensavers"
+msgstr ""
+
+#: data/xfce4-screensaver.directory.desktop.in:5
+msgid "Screensaver themes"
+msgstr ""
+
 #: savers/floaters.c:89
 msgid "Show paths that images follow"
 msgstr ""