libmate-panel-applet/Makefile.am: Fix build with gettext 0.22

With gettext-0.22 we are getting build errors like:
/usr/bin/msgfmt --desktop --keyword Name --keyword Description --template org.mate.panel.TestApplet.mate-panel-applet.desktop.in -d ../po -o org.mate.panel.TestApplet.mate-panel-applet
/usr/bin/msgfmt: no input file should be given if --desktop and -d are specified
Try '/usr/bin/msgfmt --help' for more information.
make[3]: *** [Makefile:1035: org.mate.panel.TestApplet.mate-panel-applet] Error 1
make[3]: *** Waiting for unfinished jobs....

Bug: https://bugs.gentoo.org/908877
Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>

libmate-panel-applet/Makefile.am

@@ -95,7 +95,7 @@ appletdir       = $(datadir)/mate-panel/applets
 applet_in_files = org.mate.panel.TestApplet.mate-panel-applet.desktop.in
 noinst_DATA     = $(applet_in_files:.mate-panel-applet.desktop.in=.mate-panel-applet)
 $(noinst_DATA): $(applet_in_files)
-	$(AM_V_GEN) $(MSGFMT) --desktop --keyword Name --keyword Description --template $< -d $(top_srcdir)/po -o $@
+	$(AM_V_GEN) $(MSGFMT) --desktop --keyword=Name --keyword=Description --template $< -d $(top_srcdir)/po -o $@
 
 EXTRA_DIST =						\
 	org.mate.panel.TestApplet.mate-panel-applet.desktop.in	\