Fix gtk-3.20 makefile
Fix gtk-3.20 makefile

Add the missing uninstall-local rules to the newly added
gtk-3.20 Makefile.am file.

This caused distuninstallcheck to fail.

Signed-off-by: Martin Abente Lahaye <8bfe93bb895fa79fd0e59550af8c97ff07863991@sugarlabs.org>

sugarlabs/sugar-artwork,i5o/sugar-artwork,i5o/sugar-artwork,sugarlabs/sugar-artwork,sugarlabs/sugar-artwork,i5o/sugar-artwork