slang: fix static link with readline

slang: fix static link with readline

slang forgets to link with ncurses that is a dependency of readline when
linking the slsh binary. Correct this.

While at it, also make sure to use staging ncurses5-config script and not the
host one. slang uses ncurses5-config to find terminfo location, and only
staging ncurses5-config provides the correct run-time location.

Fixes:
http://autobuild.buildroot.net/results/1dc/1dc52048254c32a24070fef1c1039cebb32e7ac1/
http://autobuild.buildroot.net/results/c17/c176511cbb147c2d8cb0ec0ff3c1612ce8971cb8/
http://autobuild.buildroot.net/results/78d/78dc2ba07b1d7f888aab94e223f3e0b1a1df3af5/

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