libssh2: fix static linking issue with libgcrypt

libssh2: fix static linking issue with libgcrypt

Fixes:
http://autobuild.buildroot.net/results/07e/07e50ba575b305b37e7a94a773c2b2e255393e9f/
http://autobuild.buildroot.net/results/072/07249f0a001a34ae608a8f8646b26a1152219252/
http://autobuild.buildroot.net/results/1c0/1c0447431e4c025732ea2eb8e9788e303f24fd04/
http://autobuild.buildroot.net/results/5e1/5e1f58c086e437b85b0d31310a93a3b2d0d70836/

And many more.

libgcrypt depends on libgpg-error and the configure script forgets to link
against it breaking static linking.  Fix it by using libgcrypt-config
--libs.

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