meson: Drop unused argument for i18n.merge_file()

Ignored in Meson < 0.60.0, deprecated since 0.60.1 and fatal since 0.61.0.

data/meson.build:23:0: ERROR: Function does not take positional arguments.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/210>

data/meson.build

@@ -20,7 +20,7 @@ foreach name: session_desktops
         session_instdir = xsessiondir
         #session_instdir = [ xesssiondir, wlsessiondir ]
     endif
-    i18n.merge_file('',
+    i18n.merge_file(
       input:  session_desktop + '.in',
       output: session_desktop,
       po_dir: '../po',