po: update POTFILES

xgettext: error while opening "gst/parse/grammar.y" for reading: No such file or directory

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/586

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/556>

po/POTFILES

@@ -9,7 +9,7 @@ gst/gstregistry.c
 gst/gsttaglist.c
 gst/gsturi.c
 gst/gstutils.c
-gst/parse/grammar.y
+gst/parse/grammar.y.in
 libs/gst/base/gstbasesink.c
 libs/gst/base/gstbasesrc.c
 plugins/elements/gstcapsfilter.c