kconfig: Support building mconf with vendor sysroot ncurses

Changes the final fallback path in the ncurses locator for mconf
to support host compilers with a non-default sysroot.

This is similar to the hardcoded search for ncurses under
'/usr/include', but can support compilers that keep their default
header and library directories elsewhere.

For nconfig, do nothing because the only vendor compiler I'm aware
of with this layout (Apple Clang) ships an ncurses version that's too
old for nconfig anyway.

Signed-off-by: John Millikin <john@john-millikin.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>