libssh: needs shared library support

libssh builds libssh.so unconditionally.

Fixes:
http://autobuild.buildroot.net/results/7c7/7c7da08c71b5378b68631e3844088307080f6f9e/
http://autobuild.buildroot.net/results/a19/a19c619f4561f5426236b958f09cab4c095a63cc/
http://autobuild.buildroot.net/results/d6d/d6dd4a9b787d654b4173c03174a2fd72bdc017b3/



and more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>