dash: fix static link with libedit

libedit is an optional dependency of dash since commit 651af57c
(dash: enable line editting if libedit is selected). Unfortunately, the
dash build system does not take into account the ncurses dependency of
libedit. Use pkgconf to get that right.

Fixes:
http://autobuild.buildroot.net/results/801/801e8bc06277978556a301ddf54f609169de37bb/
http://autobuild.buildroot.net/results/123/12367267735b9165afef77289460c247aaad494a/
http://autobuild.buildroot.net/results/aa7/aa7cc7315d778c15c5165ca6423257a84b957482/



Cc: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>