libssh2: fix link with libgcrypt

libssh2 uses the implicit 'yes' argument of the --with-libgcrypt parameter as
a library path prefix, which breaks the build. Pass the library path as
--with-libgcrypt argument to fix that. Drop the unneeded
--with-libgcrypt-prefix.

Fixes:
http://autobuild.buildroot.net/results/ac3/ac3a3f5871a6c20f5407a468809682ce491b9cb3/
http://autobuild.buildroot.net/results/ba3/ba31fa11b2ce2875649414dd9093e039dfc129e8/
http://autobuild.buildroot.net/results/b07/b07bbbf7fe164ba6dc435de6d8b41fadff7a3b08/



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