build: Always pass -no-undefined to libtool.

* src/Makefile.am: Remove no_undefined macro.

This helps to build on Cygwin and does the right thing anyway because
we do not have any dependencies in gpgrt. Thanks to Michael
Haubenwaller to explain this and propose a patch.

• GnuPG-bug-id: T4474
• Signed-off-by: Werner Koch <wk@gnupg.org>