kconfig: fix check-lxdialog for DLL platforms

Import libraries on Cygwin and MinGW/MSYS use the .dll.a suffix, so
checking this suffix is necessary to make sure ncurses will still be
found when built without static libraries.

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>