install/ui/build: Build core.js

The make-ui.sh script builds both app.js and core.js,
but only one was specified in the Makefile.
Correct the mistake.

Reviewed-By: Petr Vobornik <pvoborni@redhat.com>

install/ui/build/freeipa/Makefile.am

@@ -21,3 +21,5 @@ nav := $(wildcard ../../src/freeipa/navigation/*.js)
 
 app.js: $(core) $(base) $(widgets) $(nav)
 	../../util/make-ui.sh
+
+core.js: app.js