slang: use correct ncurses config script name

slang: use correct ncurses config script name

The ncurses config script can have different names depending on the
configuration (e.g. wchar support enabled/disabled). So, use
$(NCURSES_CONFIG_SCRIPTS) from the ncurses package as it contains the
proper name.

Fixes:

  http://autobuild.buildroot.net/results/c97554c05e121c76407be53ba37924cfcb658e9d/
  http://autobuild.buildroot.net/results/b0a31a3491a30ee0d6e6e7dbba68a86f0e00943d/
  http://autobuild.buildroot.net/results/a77241141cf8710fa945523ef62327798e3c4824/
  [...]

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>