readline: use normal/default install target

Since the custom INSTALL_TARGET doesn't consider static builds it breaks
horribly and there's no need with the current version since it works
just fine with a little varnish to remove samples. Fixes:
http://autobuild.buildroot.net/results/5a6/5a62c38ec621d49230d76981db6024035e88804a/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>