nftables: fix build in static scenarios

When linking against readline, it forgets to link against ncurses,
which is needed by readline. Fix this by passing LIBS="-lcurses" to
the configure script.

Fixes:

  http://autobuild.buildroot.org/results/98b/98b707ffdeeb1cda94b7c1019ef29cf5fd7db8bf/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>