fix(check-desktop-strings): enforce forward compatibility with gettex…

…t 0.21 behavior

Fixes #7007.  Beginning in gettext 0.21---specifically, as of
autotools-mirror/gettext@1c9b721---xgettext and (of interest to us)
msgfmt do not localize the "Icon" keyword.  That's fine, indeed
preferable, for our purposes.  But since Ubuntu Focal only has gettext
0.19, where they do, we need to pass an explicit "--keyword=Name" here
to get consistent output across platforms.

(cherry picked from commit 2e6b905)

Makefile

@@ -415,6 +415,7 @@ $(DESKTOP_POT): ${DESKTOP_BASE}/*.in
 	@msgfmt \
 		-d ${DESKTOP_LOCALE_DIR} \
 		--desktop \
+		--keyword=Name \
 		--template $< \
 		--output-file $@
 	@rm ${DESKTOP_LOCALE_DIR}/*.po