build: Fix rule to create .service files

It was always taking the first .service.in file to create all .service
files: org.gnome.Shell.CalendarServer.service.in was the only one being
used.

https://bugzilla.gnome.org/show_bug.cgi?id=659194

src/Makefile.am

@@ -19,7 +19,7 @@ typelib_DATA = $(INTROSPECTION_GIRS:.gir=.typelib)
 servicedir = $(datadir)/dbus-1/services
 service_DATA = $(service_in_files:.service.in=.service)
 
-$(service_DATA): $(service_in_files) Makefile
+%.service: %.service.in Makefile
 	$(AM_V_GEN)									\
 		[ -d $(@D) ] || $(mkdir_p) $(@D) ;					\
 		sed -e "s|\@libexecdir\@|$(libexecdir)|" $< > $@.tmp && mv $@.tmp $@