package/libssh2: fix zlib library search path issues

Like for lingcrypt and openssl, help the configure script to find
zlib installed in STAGING_DIR.

Otherwise, It might find the one installed on the host:
checking how to link with libz... /usr/lib/libz.so -Wl,-rpath -Wl,/usr/lib

Fixes:
http://autobuild.buildroot.net/results/93b/93b43e114f21a22f0f8b7d7dd6774c089c426cd1



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>