freerdp: needs shared library

Fixes:
http://autobuild.buildroot.net/results/e5d/e5dee5220cbda69e4085cbddc842c7512d500163/
http://autobuild.buildroot.net/results/edf/edf03933caa8dc35cc42bbdd14cd83c1fb952a4f/
http://autobuild.buildroot.net/results/4cf/4cf8b172f7d662aa46d359ed56572afbdccf1d85/
http://autobuild.buildroot.net/results/d39/d3922966e951f929615654e58c36bc05775fa22a/



And many more.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>